Blue Sky Energy Solutions


SOLAR Industry Terms & Definitions


absorber — In a photovoltaic device, the material that readily absorbs photons to generate charge carriers (free electrons or holes).

AC — See alternating current

acceptor — A dopant material, such as boron, which has fewer outer shell electrons than required in an otherwise balanced crystal structure, providing a hole, which can accept a free electron.

activated shelf life — The period of time, at a specified temperature, that a charged battery can be stored before its capacity falls to an unusable level.

activation voltage(s) — The voltage(s) at which a charge controller will take action to protect the batteries.

adjustable set point — A feature allowing the user to adjust the voltage levels at which a charge controller will become active.

acceptor — A dopant material, such as boron, which has fewer outer shell electrons than required in an otherwise balanced crystal structure, providing a hole, which can accept a free electron.

AIC — See amperage interrupt capability.

air mass (sometimes called air mass ratio) — Equal to the cosine of the zenith angle-that angle from directly overhead to a line intersecting the sun. The air mass is an indication of the length of the path solar radiation travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness).

alternating current (AC) — A type of electrical current, the direction of which is reversed at regular intervals or cycles. In the United States, the standard is 120 reversals or 60 cycles per second. Electricity transmission networks use AC because voltage can be controlled with relative ease.

ambient temperature — The temperature of the surrounding area.

amorphous semiconductor — A non-crystalline semiconductor material that has no long-range order.

amorphous silicon — A thin-filmsilicon photovoltaic cell having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrateSee also single-crystal silicon an polycrystalline silicon.

amperage interrupt capability (AIC) — direct current fuses should be rated with a sufficient AIC to interrupt the highest possible current.

ampere (amp) — A unit of electrical current or rate of flow of electrons. One volt across one ohm of resistance causes a current flow of one ampere.

ampere-hour (Ah/AH) — A measure of the flow of current (in amperes) over one hour; used to measure battery capacity.

ampere hour meter — An instrument that monitors current with time. The indication is the product of current (in amperes) and time (in hours).

ancillary services — Services that assist the grid operator in maintaining system balance. These include regulation and the contingency reserves: spinning, non-spinning, and in some regions, supplemental operating reserve.

angle of incidence — The angle that a ray of sun makes with a line perpendicular to the surface. For example, a surface that directly faces the sun has a solar angle of incidence of zero, but if the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90°.

annual solar savings — The annual solar savings of a solar building is the energy savings attributable to a solar feature relative to the energy requirements of a non-solar building.

anode — The positive electrode in an electrochemical cell (battery). Also, the earth or ground in a cathodic protection system. Also, the positive terminal of a diode.

antireflection coating — A thin coating of a material applied to a solar cell surface that reduces the light reflection and increases light transmission.

array — See photovoltaic (PV) array.

array current — The electrical current produced by a photovoltaic array when it is exposed to sunlight.

array operating voltage — The voltage produced by a photovoltaic array when exposed to sunlight and connected to a load.

autonomous system — See stand-alone system.

availability — The quality or condition of a photovoltaic system being available to provide power to a load. Usually measured in hours per year. One minus availability equals downtime.

azimuth angle — The angle between true south and the point on the horizon directly below the sun.


balance of system — Represents all components and costs other than the photovoltaic modules/array. It includes design costs, land, site preparation, system installation, support structures, power conditioning, operation and maintenance costs, indirect storage, and related costs.

balancing area — A metered segment of the power system, maintained by a balancing area authority, that ensures the total of all electrical generation equals the total of all system loads.

band gap — In a semiconductor, the energy difference between the highest valence band and the lowest conduction band.

band gap energy (Eg) — The amount of energy (in electron volts) required to free an outer shell electron from its orbit about the nucleus to a free state, and thus promote it from the valence to the conduction level.

barrier energy — The energy given up by an electron in penetrating the cell barrier; a measure of the electrostatic potential of the barrier.

base load — The average amount of electric power that a utility must supply in any period.

base load generating plants — Typically coal or nuclear generating units that are committed and dispatched at constant or near-constant levels with minimum cycling. They are often the sources of lowest-cost of energy when run at very high capacity factors.

battery — Two or more electrochemical cells enclosed in a container and electrically interconnected in an appropriate series/parallel arrangement to provide the required operating voltage and current levels. Under common usage, the term battery also applies to a single cell if it constitutes the entire electrochemical storage system.

battery available capacity — The total maximum charge, expressed in ampere-hours, that can be withdrawn from a cell or battery under a specific set of operating conditions including discharge rate, temperature, initial state of charge, age, and cut-off voltage.

battery capacity — The maximum total electrical charge, expressed in ampere-hours, which a battery can deliver to a load under a specific set of conditions.

battery cell — The simplest operating unit in a storage battery. It consists of one or more positive electrodes or plates, an electrolyte that permits ionic conduction, one or more negative electrodes or plates, separators between plates of opposite polarity, and a container for all the above.

battery cycle life — The number of cycles, to a specified depth of discharge, that a cell or battery can undergo before failing to meet its specified capacity or efficiency performance criteria.

battery energy capacity — The total energy available, expressed in watt-hours (kilowatt-hours), which can be withdrawn from a fully charged cell or battery. The energy capacity of a given cell varies with temperature, rate, age, and cut-off voltage. This term is more common to system designers than it is to the battery industry where capacity usually refers to ampere-hours.

battery energy storage — Energy storage using electrochemical batteries. The three main applications for battery energy storage systems include spinning reserve at generating stations, load leveling at substations, and peak shaving on the customer side of the meter.

battery life — The period during which a cell or battery is capable of operating above a specified capacity or efficiency performance level. Life may be measured in cycles and/or years, depending on the type of service for which the cell or battery is intended.

BIPV — See building integrated photovoltaics.

blocking diode — A semiconductor connected in series with a solar cell or cells and a storage battery to keep the battery from discharging through the cell when there is no output, or low output, from the solar cell. It can be thought of as a one-way valve that allows electrons to flow forwards, but not backwards.

boron (B) — The chemical element commonly used as the dopant in photovoltaic device or cell material.

boule — A sausage-shaped, synthetic single-crystal mass grown in a special furnace, pulled and turned at a rate necessary to maintain the single-crystal structure during growth.

British thermal unit (Btu) — The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.

building integrated photovoltaics — A term for the design and integration of photovoltaic (PV) technology into the building envelope, typically replacing conventional building materials. This integration may be in vertical facades, replacing view glass, spandrel glass, or other facade material; into semitransparent skylight systems; into roofing systems, replacing traditional roofing materials; into shading “eyebrows” over windows; or other building envelope systems.

bypass diode — A diode connected across one or more solar cells in a photovoltaic module such that the diode will conduct if the cell(s) become reverse biased. It protects these solar cells from thermal destruction in case of total or partial shading of individual solar cells while other cells are exposed to full light.

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cadmium (Cd) — A chemical element used in making certain types of solar cells and batteries.

cadmium telluride (CdTe) — A polycrystalline thin-film photovoltaic material.

capacity (C) — See battery capacity.

capacity factor — The ratio of the average load on (or power output of) an electricity generating unit or system to the capacity rating of the unit or system over a specified period of time.

captive electrolyte battery — A battery having an immobilized electrolyte (gelled or absorbed in a material).

cathode — The negative pole or electrode of an electrolytic cell, vacuum tube, etc., where electrons enter (current leaves) the system; the opposite of an anode.

cathodic protection — A method of preventing oxidation of the exposed metal in structures by imposing a small electrical voltage between the structure and the ground.

Cd — See cadmium.

CdTe — See cadmium telluride.

cell (battery) — A single unit of an electrochemical device capable of producing direct voltage by converting chemical energy into electrical energy. A battery usually consists of several cells electrically connected together to produce higher voltages. (Sometimes the terms cell and battery are used interchangeably). See also photovoltaic (PV) cell.

cell barrier — A very thin region of static electric charge along the interface of the positive and negative layers in a photovoltaic cell. The barrier inhibits the movement of electrons from one layer to the other, so that higher-energy electrons from one side diffuse preferentially through it in one direction, creating a current and thus a voltage across the cell. Also called depletion zone or space charge.

cell junction — The area of immediate contact between two layers (positive and negative) of a photovoltaic cell. The junction lies at the center of the cell barrier or depletion zone.

charge — The process of adding electrical energy to a battery.

charge carrier — A free and mobile conduction electron or hole in a semiconductor.

charge controller — A component of a photovoltaic system that controls the flow of current to and from the battery to protect it from over-charge and over-discharge. The charge controller may also indicate the system operational status.

charge factor — A number representing the time in hours during which a battery can be charged at a constant current without damage to the battery. Usually expressed in relation to the total battery capacity, i.e., C/5 indicates a charge factor of 5 hours. Related to charge rate.

charge rate — The current applied to a cell or battery to restore its available capacity. This rate is commonly normalized by a charge control device with respect to the rated capacity of the cell or battery.

chemical vapor deposition (CVD) — A method of depositing thin semiconductor films used to make certain types of photovoltaic devices. With this method, a substrate is exposed to one or more vaporized compounds, one or more of which contain desirable constituents. A chemical reaction is initiated, at or near the substrate surface, to produce the desired material that will condense on the substrate.

cleavage of lateral epitaxial films for transfer (CLEFT) — A process for making inexpensive gallium arsenide (GaAs) photovoltaic cells in which a thin film of GaAs is grown atop a thick, single-crystal GaAs (or other suitable material) substrate and then is cleaved from the substrate and incorporated into a cell, allowing the substrate to be reused to grow more thin-film GaAs.

cloud enhancement — The increase in solar intensity caused by reflected irradiance from nearby clouds.

combined collector — A photovoltaic device or module that provides useful heat energy in addition to electricity.

concentrating photovoltaics (CPV) — A solar technology that uses lenses or mirrors to concentrate sunlight onto high-efficiency solar cells.

concentrating solar power (CSP) — A solar technology that use mirrors to reflect and concentrate sunlight onto receivers that convert solar energy to heat. This thermal energy is then used to produce electricity with a steam turbine or heat engine driving a generator.

concentrator — A photovoltaic module, which includes optical components such as lenses (Fresnel lens) to direct and concentrate sunlight onto a solar cell of smaller area. Most concentrator arrays must directly face or track the sun. They can increase the power flux of sunlight hundreds of times.

conduction band (or conduction level) — An energy band in a semiconductor in which electrons can move freely in a solid, producing a net transport of charge.

conductor — The material through which electricity is transmitted, such as an electrical wire, or transmission or distribution line.

contact resistance — The resistance between metallic contacts and the semiconductor.

contingency reserves — Reserve services that are sufficient to cover the unplanned trip (disconnect) of a large generator or transmission line and maintain system balance. Contingency reserves are generally split between spinning and non-spinning reserves, and are often based on the largest single hazard (generator or transmission capacity).

conversion efficiency — See photovoltaic (conversion) efficiency.

converter — A unit that converts a direct current (dc) voltage to another dc voltage.

copper indium diselenide (CuInSe2, or CIS) — A polycrystalline thin-film photovoltaic material (sometimes incorporating gallium (CIGS) and/or sulfur).

copper zinc tin sulfide/selenide (CZTS) — A polycrystalline thin-film photovoltaic material.

crystalline silicon — A type of photovoltaic cell made from a slice of single-crystal silicon or polycrystalline silicon.

current — See electric current.

current at maximum power (Imp) — The current at which maximum power is available from a module.

current-voltage (I-V) curve — See I-V curve

cutoff voltage — The voltage levels (activation) at which the charge controller disconnects the photovoltaic array from the battery or the load from the battery.

cycle — The discharge and subsequent charge of a battery.

Czochralski process — A method of growing large size, high quality semiconductor crystal by slowly lifting a seed crystal from a molten bath of the material under careful cooling conditions.


dangling bonds — A chemical bond associated with an atom on the surface layer of a crystal. The bond does not join with another atom of the crystal, but extends in the direction of exterior of the surface.

days of storage — The number of consecutive days the stand-alone system will meet a defined load without solar energy input. This term is related to system availability.

DC — See direct current.

DC-to-DC converter — Electronic circuit to convert direct current voltages (e.g., photovoltaic module voltage) into other levels (e.g., load voltage). Can be part of a maximum power point tracker.

deep-cycle battery — A battery with large plates that can withstand many discharges to a low state-of-charge.

deep discharge — Discharging a battery to 20% or less of its full charge capacity.

defect — See light-induced defects

demand response — The process of using voluntary load reductions during peak hours.

depth of discharge (DOD) — The ampere-hours removed from a fully charged cell or battery, expressed as a percentage of rated capacity. For example, the removal of 25 ampere-hours from a fully charged 100 ampere-hours rated cell results in a 25% depth of discharge. Under certain conditions, such as discharge rates lower than that used to rate the cell, depth of discharge can exceed 100%.

dendrite — A slender threadlike spike of pure crystalline material, such as silicon.

dendritic web technique — A method for making sheets of polycrystalline silicon in which silicon dendrites are slowly withdrawn from a melt of silicon whereupon a web of silicon forms between the dendrites and solidifies as it rises from the melt and cools.

depletion zone — Same as cell barrier. The term derives from the fact that this microscopically thin region is depleted of charge carriers (free electrons and hole).

design month — The month having the combination of insolation and load that requires the maximum energy from the photovoltaic array.

diffuse insolation — Sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere. Opposite of direct insolation.

diffuse radiation — Radiation received from the sun after reflection and scattering by the atmosphere and ground.

diffusion furnace — Furnace used to make junctions in semiconductors by diffusing dopant atoms into the surface of the material.

diffusion length — The mean distance a free electron or hole moves before recombining with another hole or electron.

diode — An electronic device that allows current to flow in one direction only. See also blocking diode and bypass diode.

direct beam radiation — Radiation received by direct solar rays. Measured by a pyrheliometer with a solar aperture of 5.7° to transcribe the solar disc.

direct current (DC) — A type of electricity transmission and distribution by which electricity flows in one direction through the conductor, usually relatively low voltage and high current. To be used for typical 120 volt or 220 volt household appliances, DC must be converted to alternating current, its opposite.

direct insolation — Sunlight falling directly upon a collector. Opposite of diffuse insolation.

discharge — The withdrawal of electrical energy from a battery.

discharge factor — A number equivalent to the time in hours during which a battery is discharged at constant current usually expressed as a percentage of the total battery capacity, i.e., C/5 indicates a discharge factor of 5 hours. Related to discharge rate.

discharge rate — The rate, usually expressed in amperes or time, at which electrical current is taken from the battery.

disconnect — Switch gear used to connect or disconnect components in a photovoltaic system.

dispatching (economic dispatch) — A method by which system operators decide how much output should be scheduled from plants.

distributed energy resources (DER) — A variety of small, modular power-generating technologies that can be combined with energy management and storage systems and used to improve the operation of the electricity delivery system, whether or not those technologies are connected to an electricity grid.

distributed generation — A popular term for localized or on-site power generation.

distributed power — Generic term for any power supply located near the point where the power is used. Opposite of central power. See also stand-alone systems.

distributed systems — Systems that are installed at or near the location where the electricity is used, as opposed to central systems that supply electricity to grids. A residential photovoltaic system is a distributed system.

donor — In a photovoltaic device, an n-type dopant, such as phosphorus, that puts an additional electron into an energy level very near the conduction band; this electron is easily exited into the conduction band where it increases the electrical conductivity over than of an undoped semiconductor.

donor level — The level that donates conduction electrons to the system.

dopant — A chemical element (impurity) added in small amounts to an otherwise pure semiconductor material to modify the electrical properties of the material. An n-dopant introduces more electrons. A p-dopant creates electron vacancies (holes).

doping — The addition of dopants to a semiconductor.

downtime — Time when the photovoltaic system cannot provide power for the load. Usually expressed in hours per year or that percentage.

dry cell — A cell (battery) with a captive electrolyte. A primary battery that cannot be recharged.

duty cycle — The ratio of active time to total time. Used to describe the operating regime of appliances or loads in photovoltaic systems.

duty rating — The amount of time an inverter (power conditioning unit) can produce at full rated power.


edge-defined film-fed growth (EFG) — A method for making sheets of polycrystalline silicon for photovoltaic devices in which molten silicon is drawn upward by capillary action through a mold.

electric circuit — The path followed by electrons from a power source (generator or battery), through an electrical system, and returning to the source.

electric current — The flow of electrical energy (electricity) in a conductor, measured in amperes.

electrical grid — An integrated system of electricity distribution, usually covering a large area.

electricity — Energy resulting from the flow of charge particles, such as electrons or ions.

electrochemical cell — A device containing two conducting electrodes, one positive and the other negative, made of dissimilar materials (usually metals) that are immersed in a chemical solution (electrolyte) that transmits positive ions from the negative to the positive electrode and thus forms an electrical charge. One or more cells constitute a battery.

electrode — A conductor that is brought in conducting contact with a ground.

electrodeposition — Electrolytic process in which a metal is deposited at the cathode from a solution of its ions.

electrolyte — A nonmetallic (liquid or solid) conductor that carries current by the movement of ions (instead of electrons) with the liberation of matter at the electrodes of an electrochemical cell.

electron — An elementary particle of an atom with a negative electrical charge and a mass of 1/1837 of a proton; electrons surround the positively charged nucleus of an atom and determine the chemical properties of an atom. The movement of electrons in an electrical conductor constitutes an electric current.

electron hole pair — The result of light of sufficient energy dislodging an electron from its bond in a crystal, which creates a hole. The free electron (negative charge) and the hole (positive charge) are a pair. These pairs are the constituents of electricity.

electron volt (eV) — The amount of kinetic energy gained by an electron when accelerated through an electric potential difference of 1 Volt; equivalent to 1.603 x 10^-19; a unit of energy or work.

energy — The capability of doing work; different forms of energy can be converted to other forms, but the total amount of energy remains the same.

energy audit — A survey that shows how much energy used in a home, which helps find ways to use less energy.

energy contribution potential — Recombination occurring in the emitter region of a photovoltaic cell.

energy density — The ratio of available energy per pound; usually used to compare storage batteries.

energy imbalance service — A market service that provides for the management of unscheduled deviations in individual generator output or load consumption.

energy levels — The energy represented by an electron in the band model of a substance.

epitaxial growth — The growth of one crystal on the surface of another crystal. The growth of the deposited crystal is oriented by the lattice structure of the original crystal.

equalization — The process of restoring all cells in a battery to an equal state-of-charge. Some battery types may require a complete discharge as a part of the equalization process.

equalization charge — The process of mixing the electrolyte in batteries by periodically overcharging the batteries for a short time.

equalizing charge — A continuation of normal battery charging, at a voltage level slightly higher than the normal end-of-charge voltage, in order to provide cell equalization within a battery.

equinox — The two times of the year when the sun crosses the equator and night and day are of equal length; occurring around March 20 or 21 (spring equinox) and September 22 or 23 (fall equinox).

exciton — A quasi-particle created in a semiconductor that is composed of an electron hole pair in a bound state. An exciton can be generated by and converted back into a photon.

external quantum efficiency (external QE or EQE) — Quantum efficiency that includes the effect of optical losses, such as transmission through the cell and reflection of light away from the cell.

extrinsic semiconductor — The product of doping a pure semiconductor.


Fermi level — Energy level at which the probability of finding an electron is one-half. In a metal, the Fermi level is very near the top of the filled levels in the partially filled valence band. In a semiconductor, the Fermi level is in the band gap.

fill factor — The ratio of a photovoltaic cell‘s actual power to its power if both current and voltage were at their maxima. A key characteristic in evaluating cell performance.

fixed tilt array — A photovoltaic array set in at a fixed angle with respect to horizontal.

flat-plate array — A photovoltaic (PV) array that consists of non-concentrating PV modules.

flat-plate module — An arrangement of photovoltaic cells or material mounted on a rigid flat surface with the cells exposed freely to incoming sunlight.

flat-plate photovoltaics (PV) — A PV array or module that consists of nonconcentrating elements. Flat-plate arrays and modules use direct and diffuse sunlight, but if the array is fixed in position, some portion of the direct sunlight is lost because of oblique sun-angles in relation to the array.

float charge — The voltage required to counteract the self-discharge of the battery at a certain temperature.

float life — The number of years that a battery can keep its stated capacity when it is kept at float charge.

float service — A battery operation in which the battery is normally connected to an external current source; for instance, a battery charger which supplies the battery load< under normal conditions, while also providing enough energy input to the battery to make up for its internal quiescent losses, thus keeping the battery always up to full power and ready for service.

float-zone process — In reference to solar photovoltaic cell manufacture, a method of growing a large-size, high-quality crystal whereby coils heat a polycrystalline ingot placed atop a single-crystal seed. As the coils are slowly raised the molten interface beneath the coils becomes single crystal.

frequency — The number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz).

frequency regulation — This indicates the variability in the output frequency. Some loads will switch off or not operate properly if frequency variations exceed 1%.

Fresnel lens — An optical device that focuses light like a magnifying glass; concentric rings are faced at slightly different angles so that light falling on any ring is focused to the same point.

full sun — The amount of power density in sunlight received at the earth’s surface at noon on a clear day (about 1,000 Watts/square meter).


Ga — See gallium.

GaAs — See gallium arsenide.

gallium (Ga) — A chemical element, metallic in nature, used in making certain kinds of solar cells and semiconductor devices.

gallium arsenide (GaAs) — A crystalline, high-efficiency compound used to make certain types of solar cells and semiconductor material.

gassing — The evolution of gas from one or more of the electrodes in the cells of a battery. Gassing commonly results from local action self-discharge or from the electrolysis of water in the electrolyte during charging.

gassing current — The portion of charge current that goes into electrolytical production of hydrogen and oxygen from the electrolytic liquid. This current increases with increasing voltage and temperature.

gel-type battery — Lead-acid battery in which the electrolyte is composed of a silica gel matrix.

gigawatt (GW) — A unit of power equal to 1 billion Watts; 1 million kilowatts, or 1,000 megawatts.

grid — See electrical grid.

grid-connected system — A solar electric or photovoltaic (PV) system in which the PV array acts like a central generating plant, supplying power to the grid.

grid-interactive system — Same as grid-connected system.

grid lines — Metallic contacts fused to the surface of the solar cell to provide a low resistance path for electrons to flow out to the cell interconnect wires.


harmonic content — The number of frequencies in the output waveform in addition to the primary frequency (50 or 60 Hz.). Energy in these harmonic frequencies is lost and may cause excessive heating of the load.

heterojunction — A region of electrical contact between two different materials.

high voltage disconnect — The voltage at which a charge controller will disconnect the photovoltaic array from the batteries to prevent overcharging.

high voltage disconnect hysteresis — The voltage difference between the high voltag disconnect set point and the voltage at which the full photovoltaic array current will be reapplied.

hole — The vacancy where an electron would normally exist in a solid; behaves like a positively charged particle.

homojunction — The region between an n-layer and a p-layer in a single material, photovoltaic cell.

hybrid system — A solar electric or photovoltaic system that includes other sources of electricity generation, such as wind or diesel generators.

hydrogenated amorphous silicon — Amorphous silicon with a small amount of incorporated hydrogen. The hydrogen neutralizes dangling bonds in the amorphous silicon, allowing charge carriers to flow more freely.


incident light — Light that shines onto the face of a solar cell or module.

independent system operator (ISO) — The entity responsible for maintaining system balance, reliability, and electricity market operation.

indium oxide — A wide band gap semiconductor that can be heavily doped with tin to make a highly conductive, transparent thin film. Often used as a front contact or one component of a heterojunction solar cell.

infrared radiation — Electromagnetic radiation whose wavelengths lie in the range from 0.75 micrometer to 1000 micrometers; invisible long wavelength radiation (heat) capable of producing a thermal or photovoltaic effect, though less effective than visible light.

ingot — A casting of material, usually crystalline silicon, from which slices or wafers can be cut for use in a solar cell.

input voltage — This is determined by the total power required by the alternating current loads and the voltage of any direct current loads. Generally, the larger the load, the higher the inverter input voltage. This keeps the current at levels where switches and other components are readily available.

insolation — The solar power density incident on a surface of stated area and orientation, usually expressed as Watts per square meter or Btu per square foot per hour. See also diffuse insolation and direct insolation.

interconnect — A conductor within a module or other means of connection that provides an electrical interconnection between the solar cells.

internal quantum efficiency (internal QE or IQE) — A type of quantum efficiency. Refers to the efficiency with which light not transmitted through or reflected away from the cell can generate charge carriers that can generate current.

intrinsic layer — A layer of semiconductor material, used in a photovoltaic device, whose properties are essentially those of the pure, undoped, material.

intrinsic semiconductor — An undoped semiconductor.

inverted metamorphic multijunction (IMM) cell — A photovoltaic cell that is a multijunction device whose layers of semiconductors are grown upside down. This special manufacturing process yields an ultra-light and flexible cell that also converts solar energy with high efficiency.

inverter — A device that converts direct current electricity to alternating current either for stand-alone systems or to supply power to an electricity grid.

ion — An electrically charged atom or group of atoms that has lost or gained electrons; a loss makes the resulting particle positively charged; a gain makes the particle negatively charged.

irradiance — The direct, diffuse, and reflected solar radiation that strikes a surface. Usually expressed in kilowatts per square meter. Irradiance multiplied by time equals insolation.

ISPRA guidelines — Guidelines for the assessment of photovoltaic power plants, published by the Joint Research Centre of the Commission of the European Communities, Ispra, Italy.

i-type semiconductor — Semiconductor material that is left intrinsic, or undoped so that the concentration of charge carriers is characteristic of the material itself rather than of added impurities.

I-V curve — A graphical presentation of the current (I) versus the voltage (V) from a photovoltaic device as the load is increased from the short circuit (no load) condition to the open circuit (maximum voltage) condition. The shape of the curve characterizes cell performance.


joule — A metric unit of energy or work; 1 joule per second equals 1 watt or 0.737 foot-pounds; 1 Btu equals 1,055 joules.

junction — A region of transition between semiconductor layers, such as a p/n junction, which goes from a region that has a high concentration of acceptors (p-type) to one that has a high concentration of donors (n-type).

junction box — A photovoltaic (PV) generator junction box is an enclosure on the module where PV strings are electrically connected and where protection devices can be located, if necessary.

junction diode — A semiconductor device with a junction and a built-in potential that passes current better in one direction than the other. All solar cells are junction diodes.


kerf — The width of a cut used to create wafers from silicon ingots, often resulting in the loss of semiconductor material.

kilowatt (kW) — A standard unit of electrical power equal to 1000 watts, or to the energy consumption at a rate of 1000 joules per second.

kilowatt-hour (kWh) — 1,000 thousand watts acting over a period of 1 hour. The kWh is a unit of energy. 1 kWh=3600 kJ.


langley (L) — Unit of solar irradiance. One gram calorie per square centimeter. 1 L = 85.93 kWh/m2.

lattice — The regular periodic arrangement of atoms or molecules in a crystal of semiconductor material.

lead-acid battery — A general category that includes batteries with plates made of pure lead, lead-antimony, or lead-calcium immersed in an acid electrolyte.

levelized cost of energy (LCOE) — The cost of energy of a solar system that is based on the system’s installed price, its total lifetime cost, and its lifetime electricity production.

life — The period during which a system is capable of operating above a specified performance level.

life-cycle cost — The estimated cost of owning and operating a photovoltaic system for the period of its useful life.

light-induced defects — Defects, such as dangling bonds, induced in an amorphous silicon semiconductor upon initial exposure to light.

light trapping — The trapping of light inside a semiconductor material by refracting and reflecting the light at critical angles; trapped light will travel further in the material, greatly increasing the probability of absorption and hence of producing charge carriers.

line-commutated inverter — An inverter that is tied to a power grid or line. The commutation of power (conversion from direct current to alternating current) is controlled by the power line, so that, if there is a failure in the power grid, the photovoltaic system cannot feed power into the line.

liquid electrolyte battery — A battery containing a liquid solution of acid and water. Distilled water may be added to these batteries to replenish the electrolyte as necessary. Also called a flooded battery because the plates are covered with the electrolyte.

load — The demand on an energy producing system; the energy consumption or requirement of a piece or group of equipment. Usually expressed in terms of amperes or watts in reference to electricity.

load circuit — The wire, switches, fuses, etc. that connect the load to the power source.

load current (A) — The current required by the electrical device.

load forecast — Predictions of future demand. For normal operations, daily and weekly forecasts of the hour-by-hour demand are used to help develop generation schedules to ensure that sufficient quantities and types of generation are available when needed.

load resistance — The resistance presented by the load. See also resistance.

locational marginal price (LMP) — The price of a unit of energy at a particular electrical location at a given time. LMPs are influenced by the nearby generation, load level, and transmission constraints and losses.

low voltage cutoff (LVC) — The voltage level at which a charge controller will disconnect the load from the battery.

low voltage disconnect — The voltage at which a charge controller will disconnect the load from the batteries to prevent over-discharging.

low voltage disconnect hysteresis — The voltage difference between the low voltage disconnect set point and the voltage at which the load will be reconnected.

low voltage warning — A warning buzzer or light that indicates the low battery voltage set point has been reached.


maintenance-free battery — A sealed battery to which water cannot be added to maintain electrolyte level.

majority carrier — Current carriers (either free electrons or holes) that are in excess in a specific layer of a semiconductor material (electrons in the n-layer, holes in the p-layer) of a cell.

maximum power point (MPP) — The point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage is maximum. For a typical silicon cell, this is at about 0.45 volts.

maximum power point tracker (MPPT) — Means of a power conditioning unit that automatically operates the photovoltaic generator at its maximum power point under all conditions.

maximum power tracking — Operating a photovoltaic array at the peak power point of the array’s I-V curve where maximum power is obtained. Also called peak power tracking.

measurement and characterization — A field of research that involves assessing the characteristics of photovoltaic materials and devices.

megawatt (MW) — 1,000 kilowatts, or 1 million watts; standard measure of electric power plant generating capacity.

megawatt-hour — 1,000 kilowatt-hours or 1 million watt-hours.

metrology — The science of measurement.

microgroove — A small groove scribed into the surface of a solar cell, which is filled with metal for contacts.

micrometer (micron) — One millionth of a meter.

minority carrier — A current carrier, either an electron or a hole, that is in the minority in a specific layer of a semiconductor material; the diffusion of minority carriers under the action of the cell junction voltage is the current in a photovoltaic device.

minority carrier lifetime — The average time a minority carrier exists before recombination.

modified sine wave — A waveform that has at least three states (i.e., positive, off, and negative). Has less harmonic content than a square wave.

modularity — The use of multiple inverters connected in parallel to service different loads.

module — See photovoltaic (PV) module.

module derate factor — A factor that lowers the photovoltaic module current to account for field operating conditions such as dirt accumulation on the module.

monolithic — Fabricated as a single structure.

movistor — Short for metal oxide varistor. Used to protect electronic circuits from surge currents such as those produced by lightning.

multicrystalline — A semiconductor (photovoltaic) material composed of variously oriented, small, individual crystals. Sometimes referred to as polycrystalline or semicrystalline.

multijunction device — A high-efficiency photovoltaic device containing two or more cell junctions, each of which is optimized for a particular part of the solar spectrum.

multi-stage controller — A charging controller unit that allows different charging currents as the battery nears full state_of_charge.


nanometer — One billionth of a meter.

National Electrical Code (NEC) — Contains guidelines for all types of electrical installations. The 1984 and later editions of the NEC contain Article 690, “Solar Photovoltaic Systems” which should be followed when installing a PV system.

National Electrical Manufacturers Association (NEMA) — This organization sets standards for some non-electronic products like junction boxes.

NEC — See National Electrical Code.

NEMA — See National Electrical Manufacturers Association.

nickel cadmium battery — A battery containing nickel and cadmium plates and an alkaline electrolyte.

nominal voltage — A reference voltage used to describe batteriesmodules, or systems (i.e., a 12-volt or 24-volt battery, module, or system).

normal operating cell temperature (NOCT) — The estimated temperature of a photovoltaic module when operating under 800 w/m2 irradiance, 20°C ambient temperature and wind speed of 1 meter per second. NOCT is used to estimate the nominal operating temperature of a module in its working environment.

n-type — Negative semiconductor material in which there are more electrons than holescurrent is carried through it by the flow of electrons.

n-type semiconductor — A semiconductor produced by doping an intrinsic semiconductor with an electrondonor impurity (e.g., phosphorus in silicon).

n-type silicon — Silicon material that has been doped with a material that has more electrons in its atomic structure than does silicon.


ohm — A measure of the electrical resistance of a material equal to the resistance of a circuit in which the potential difference of 1 volt produces a current of 1 ampere.

one-axis tracking — A system capable of rotating about one axis.

open-circuit voltage (Voc) — The maximum possible voltage across a photovoltaic cell; the voltage across the cell in sunlight when no current is flowing.

operating point — The current and voltage that a photovoltaic module or array produces when connected to a load. The operating point is dependent on the load or the batteries connected to the output terminals of the array.

orientation — Placement with respect to the cardinal directions, N, S, E, W; azimuth is the measure of orientation from north.

outgas — See gassing.

overcharge — Forcing current into a fully charged battery. The battery will be damaged if overcharged for a long period.


packing factor — The ratio of array area to actual land area or building envelope area for a system; or, the ratio of total solar cell area to the total module area, for a module.

panel — See photovoltaic (PV) panel.

parallel connection — A way of joining solar cells or photovoltaic modules by connecting positive leads together and negative leads together; such a configuration increases the current, but not the voltage.

passivation — A chemical reaction that eliminates the detrimental effect of electrically reactive atoms on a solar cell’s surface.

peak demand/load — The maximum energy demand or load in a specified time period.

peak power current — Amperes produced by a photovoltaic module or array operating at the voltage of the I-V curve that will produce maximum power from the module.

peak power point — Operating point of the I-V (current-voltage) curve for a solar cell or photovoltaic module where the product of the current value times the voltage value is a maximum.

peak power tracking — See maximum power tracking.

peak sun hours — The equivalent number of hours per day when solar irradiance averages 1,000 w/m2. For example, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the irradiance for six hours been 1,000 w/m2.

peak watt — A unit used to rate the performance of solar cellsmodules, or arrays; the maximum nominal output of a photovoltaic device, in watts (Wp) under standardized test conditions, usually 1,000 watts per square meter of sunlight with other conditions, such as temperature specified.

phosphorous (P) — A chemical element used as a dopant in making n-type semiconductor layers.

photocurrent — An electric current induced by radiant energy.

photoelectric cell — A device for measuring light intensity that works by converting light falling on, or reach it, to electricity, and then measuring the current; used in photometers.

photoelectrochemical cell — A type of photovoltaic device in which the electricity induced in the cell is used immediately within the cell to produce a chemical, such as hydrogen, which can then be withdrawn for use.

photon — A particle of light that acts as an individual unit of energy.

photovoltaic(s) (PV) — Pertaining to the direct conversion of light into electricity.

photovoltaic (PV) array — An interconnected system of PV modules that function as a single electricity-producing unit. The modules are assembled as a discrete structure, with common support or mounting. In smaller systems, an array can consist of a single module.

photovoltaic (PV) cell — The smallest semiconductor element within a PV module to perform the immediate conversion of light into electrical energy (direct current voltage and current). Also called a solar cell.

photovoltaic (PV) conversion efficiency — The ratio of the electric power produced by a photovoltaic device to the power of the sunlight incident on the device.

photovoltaic (PV) device — A solid-state electrical device that converts light directly into direct current electricity of voltage-current characteristics that are a function of the characteristics of the light source and the materials in and design of the device. Solar photovoltaic devices are made of various semiconductor materials including siliconcadmium sulfidecadmium telluride, and gallium arsenide, and in single crystalline, multicrystalline, or amorphous forms.

photovoltaic (PV) effect — The phenomenon that occurs when photons, the “particles” in a beam of light, knock electrons loose from the atoms they strike. When this property of light is combined with the properties of semiconductors, electrons flow in one direction across a junction, setting up a voltage. With the addition of circuitry, current will flow and electric power will be available.

photovoltaic (PV) generator — The total of all PV strings of a PV power supply system, which are electrically interconnected.

photovoltaic (PV) module — The smallest environmentally protected, essentially planar assembly of solar cells and ancillary parts, such as interconnections, terminals, (and protective devices such as diodes) intended to generate direct current power under unconcentrated sunlight. The structural (load carrying) member of a module can either be the top layer (superstrate) or the back layer (substrate).

photovoltaic (PV) panel — often used interchangeably with PV module (especially in one-module systems), but more accurately used to refer to a physically connected collection of modules (i.e., a laminate string of modules used to achieve a required voltage and current).

photovoltaic (PV) system — A complete set of components for converting sunlight into electricity by the photovoltaic process, including the array and balance of system components.

photovoltaic-thermal (PV/T) system — A photovoltaic system that, in addition to converting sunlight into electricity, collects the residual heat energy and delivers both heat and electricity in usable form. Also called a total energy system or solar thermal system.

physical vapor deposition — A method of depositing thin semiconductor photovoltaic films. With this method, physical processes, such as thermal evaporation or bombardment of ions, are used to deposit elemental semiconductor material on a substrate.

P-I-N — A semiconductor photovoltaic (PV) device structure that layers an intrinsic semiconductor between a p-type semiconductor and an n-type semiconductor; this structure is most often used with amorphous silicon PV devices.

plates — A metal plate, usually lead or lead compound, immersed in the electrolyte in a battery.

plug-and-play PV system — A commercial, off-the-shelf photovoltaic system that is fully inclusive with little need for individual customization. The system can be installed without special training and using few tools. The homeowner plugs the system into a PV-ready circuit and an automatic PV discovery process initiates communication between the system and the utility. The system and grid are automatically configured for optimal operation.

P/N — A semiconductor photovoltaic device structure in which the junction is formed between a p-type layer and an n-type layer.

pocket plate — A plate for a battery in which active materials are held in a perforated metal pocket.

point-contact cell — A high efficiency silicon photovoltaic concentrator cell that employs light trapping techniques and point-diffused contacts on the rear surface for current collection.

polycrystalline — See multicrystalline.

polycrystalline silicon — A material used to make photovoltaic cells, which consist of many crystals unlike single-crystal silicon.

polycrystalline thin film — A thin film made of multicrystalline material.

power — The amount of electrical energy available for doing work, measured in horsepower, Watts, or Btu per hour.

power conditioning — The process of modifying the characteristics of electrical power (for e.g., inverting direct current to alternating current).

power conditioning equipment — Electrical equipment, or power electronics, used to convert power from a photovoltaic array into a form suitable for subsequent use. A collective term for inverter, converter, battery charge regulator, and blocking diode.

power conversion efficiency — The ratio of output power to input power of the inverter.

power density — The ratio of the power available from a battery to its mass (W/kg) or volume (W/l).

power factor (PF) — The ratio of actual power being used in a circuit, expressed in watts or kilowatts, to the power that is apparently being drawn from a power source, expressed in volt-amperes or kilovolt-amperes.

primary battery — A battery whose initial capacity cannot be restored by charging.

projected area — The net south-facing glazing area projected on a vertical plane.

p-type semiconductor — A semiconductor in which holes carry the current; produced by doping an intrinsic semiconductor with an electron acceptor impurity (e.g., boron in silicon).

pulse-width-modulated (PWM) wave inverter — A type of power inverter that produce a high quality (nearly sinusoidal) voltage, at minimum current harmonics.

PV — See photovoltaic(s).

pyranometer — An instrument used for measuring global solar irradiance.

pyrheliometer — An instrument used for measuring direct beam solar irradiance. Uses an aperture of 5.7° to transcribe the solar disc.


quad — One quadrillion Btu (1,000,000,000,000,000 Btu).

qualification test — A procedure applied to a selected set of photovoltaic modules involving the application of defined electrical, mechanical, or thermal stress in a prescribed manner and amount. Test results are subject to a list of defined requirements.

quantum efficiency (QE) — The ratio of the number of charge carriers collected by a photovoltaic cell to the number of photons of a given energy shining on the cell. Quantum efficiency relates to the response of a solar cell to the different wavelengths in the spectrum of light shining on the cell. QE is given as a function of either wavelength or energy. Optimally, a solar cell should generate considerable electrical current for wavelengths that are most abundant in sunlight.


ramp — A change in generation output.

ramp rate — The ability of a generating unit to change its output over some unit of time, often measured in MW/min.

Rankine cycle — A thermodynamic cycle used in steam turbines to convert heat energy into work. Concentrating solar power plants often rely on the Rankine cycle. In CSP systems, mirrors focus sunlight on a heat-transfer fluid. This is used to creates steam, which spins a turbine to generate electricity.

rated battery capacity — The term used by battery manufacturers to indicate the maximum amount of energy that can be withdrawn from a battery under specified discharge rate and temperature. See also battery capacity.

rated module current (A) — The current output of a photovoltaic module measured at standard test conditions of 1,000 w/m2 and 25°C cell temperature.

rated power — Rated power of the inverter. However, some units can not produce rated power continuously. See also duty rating.

reactive power — The sine of the phase angle between the current and voltage waveforms in an alternating current system. See also power factor.

recombination — The action of a free electron falling back into a hole. Recombination processes are either radiative, where the energy of recombination results in the emission of a photon, or nonradiative, where the energy of recombination is given to a second electron which then relaxes back to its original energy by emitting phonons. Recombination can take place in the bulk of the semiconductor, at the surfaces, in the junction region, at defects, or between interfaces.

rectifier — A device that converts alternating current to direct currentSee also inverter.

regulator — Prevents overcharging of batteries by controlling charge cycle-usually adjustable to conform to specific battery needs.

remote systems — See stand-alone systems.

reserve capacity — The amount of generating capacity a central power system must maintain to meet peak loads.

resistance (R) — The property of a conductor, which opposes the flow of an electric current resulting in the generation of heat in the conducting material. The measure of the resistance of a given conductor is the electromotive force needed for a unit current flow. The unit of resistance is ohms.

resistive voltage drop — The voltage developed across a cell by the current flow through the resistance of the cell.

reverse current protection — Any method of preventing unwanted current flow from the battery to the photovoltaic array (usually at night). See also blocking diode.

ribbon (photovoltaic) cells — A type of photovoltaic device made in a continuous process of pulling material from a molten bath of photovoltaic material, such as silicon, to form a thin sheet of material.

RMS — See root mean square.

root mean square (RMS) — The square root of the average square of the instantaneous values of an ac output. For a sine wave the RMS value is 0.707 times the peak value. The equivalent value of alternating current, I, that will produce the same heating in a conductor with resistance, R, as a dc current of value I.


sacrificial anode — A piece of metal buried near a structure that is to be protected from corrosion. The metal of the sacrificial anode is intended to corrode and reduce the corrosion of the protected structure.

satellite power system (SPS) — Concept for providing large amounts of electricity for use on the Earth from one or more satellites in geosynchronous Earth orbit. A very large array of solar cells on each satellite would provide electricity, which would be converted to microwave energy and beamed to a receiving antenna on the ground. There, it would be reconverted into electricity and distributed the same as any other centrally generated power, through a grid.

scheduling — The general practice of ensuring that a generator is committed and available when needed. It also can refer to scheduling of imports or exports of energy into or out of a balancing area.

Schottky barrier — A cell barrier established as the interface between a semiconductor, such as silicon, and a sheet of metal.

scribing — The cutting of a grid pattern of grooves in a semiconductor material, generally for the purpose of making interconnections.

sealed battery — A battery with a captive electrolyte and a resealing vent cap, also called a valve-regulated battery. Electrolyte cannot be added.

seasonal depth of discharge — An adjustment factor used in some system sizing procedures which “allows” the battery to be gradually discharged over a 30-90 day period of poor solar insolation. This factor results in a slightly smaller photovoltaic array.

secondary battery — A battery that can be recharged.

self-discharge — The rate at which a battery, without a load, will lose its charge.

semiconductor — Any material that has a limited capacity for conducting an electric current. Certain semiconductors, including silicongallium arsenidecopper indium diselenide, and cadmium telluride, are uniquely suited to the photovoltaic conversion process.

semicrystalline — See multicrystalline.

series connection — A way of joining photovoltaic cells by connecting positive leads to negative leads; such a configuration increases the voltage.

series controller — A charge controller that interrupts the charging current by open-circuiting the photovoltaic (PV) array. The control element is in series with the PV array and battery.

series regulator — Type of battery charge regulator where the charging current is controlled by a switch connected in series with the photovoltaic module or array.

series resistance — Parasitic resistance to current flow in a cell due to mechanisms such as resistance from the bulk of the semiconductor material, metallic contacts, and interconnections.

shallow-cycle battery — A battery with small plates that cannot withstand many discharges to a low state-of-charge.

shelf life of batteries — The length of time, under specified conditions, that a battery can be stored so that it keeps its guaranteed capacity.

short-circuit current (Isc) — The current flowing freely through an external circuit that has no load or resistance; the maximum current possible.

shunt controller — A charge controller that redirects or shunts the charging current away from the battery. The controller requires a large heat sink to dissipate the current from the short-circuited photovoltaic array. Most shunt controllers are for smaller systems producing 30 amperes or less.

shunt regulator — Type of a battery charge regulator where the charging current is controlled by a switch connected in parallel with the photovoltaic (PV) generator. Shorting the PV generator prevents overcharging of the battery.

Siemens process — A commercial method of making purified silicon.

silicon (Si) — A semi-metallic chemical element that makes an excellent semiconductor material for photovoltaic devices. It crystallizes in face-centered cubic lattice like a diamond. It’s commonly found in sand and quartz (as the oxide).

sine wave — A waveform corresponding to a single-frequency periodic oscillation that can be mathematically represented as a function of amplitude versus angle in which the value of the curve at any point is equal to the sine of that angle.

sine wave inverter — An inverter that produces utility-quality, sine wave power forms.

single-crystal material — A material that is composed of a single crystal or a few large crystals.

single-crystal silicon — Material with a single crystalline formation. Many photovoltaic cells are made from single-crystal silicon.

single-stage controller — A charge controller that redirects all charging current as the battery nears full state-of-charge.

smart grid — An intelligent electric power system that regulates the two-way flow of electricity and information between power plants and consumers to control grid activity.

soft costs — Non-hardware costs related to PV systems, such as financing, permitting, installation, interconnection, and inspection.

solar cell — See photovoltaic (PV) cell.

solar constant — The average amount of solar radiation that reaches the earth’s upper atmosphere on a surface perpendicular to the sun’s rays; equal to 1353 watts per square meter or 492 Btu per square foot.

solar cooling — The use of solar thermal energy or solar electricity to power a cooling appliance. Photovoltaic systems can power evaporative coolers (“swamp” coolers), heat-pumps, and air conditioners.

solar energy — Electromagnetic energy transmitted from the sun (solar radiation). The amount that reaches the earth is equal to one billionth of total solar energy generated, or the equivalent of about 420 trillion kilowatt-hours.

solar-grade silicon — Intermediate-grade silicon used in the manufacture of solar cells. Less expensive than electronic-grade silicon.

solar insolation — See insolation.

solar irradiance — See irradiance.

solar noon — The time of the day, at a specific location, when the sun reaches its highest, apparent point in the sky.

solar panel — See photovoltaic (PV) panel.

solar resource — The amount of solar insolation a site receives, usually measured in kWh/m2/day, which is equivalent to the number of peak sun hours.

solar spectrum — The total distribution of electromagnetic radiation emanating from the sun. The different regions of the solar spectrum are described by their wavelength range. The visible region extends from about 390 to 780 nanometers (a nanometer is one billionth of one meter). About 99 percent of solar radiation is contained in a wavelength region from 300 nm (ultraviolet) to 3,000 nm (near-infrared). The combined radiation in the wavelength region from 280 nm to 4,000 nm is called the broadband, or total, solar radiation.

solar thermal electric systems — Solar energy conversion technologies that convert solar energy to electricity, by heating a working fluid to power a turbine that drives a generator. Examples of these systems include central receiver systems, parabolic dish, and solar trough.

space charge — See cell barrier.

specific gravity — The ratio of the weight of the solution to the weight of an equal volume of water at a specified temperature. Used as an indicator of battery state-of-charge.

spinning reserve — Electric power plant or utility capacity on-line and running at low power in excess of actual load.

split-spectrum cell — A compound photovoltaic device in which sunlight is first divided into spectral regions by optical means. Each region is then directed to a different photovoltaic cell optimized for converting that portion of the spectrum into electricity. Such a device achieves significantly greater overall conversion of incident sunlight into electricity. See also mulitjunction device.

sputtering — A process used to apply photovoltaic semiconductor material to a substrate by a physical vapor deposition process where high-energy ions are used to bombard elemental sources of semiconductor material, which eject vapors of atoms that are then deposited in thin layers on a substrate.

square wave — A waveform that has only two states, (i.e., positive or negative). A square wave contains a large number of harmonics.

square wave inverter — A type of inverter that produces square wave output. It consists of a direct current source, four switches, and the load. The switches are power semiconductors that can carry a large current and withstand a high voltage rating. The switches are turned on and off at a correct sequence, at a certain frequency.

Staebler-Wronski effect — The tendency of the sunlight to electricity conversion efficiency of amorphous silicon photovoltaic devices to degrade (drop) upon initial exposure to light.

stand-alone system — An autonomous or hybrid photovoltaic system not connected to a grid. May or may not have storage, but most stand-alone systems require batteries or some other form of storage.

standard reporting conditions (SRC) — A fixed set of conditions (including meteorological) to which the electrical performance data of a photovoltaic module are translated from the set of actual test conditions.

standard test conditions (STC) — Conditions under which a module is typically tested in a laboratory.

standby current — This is the amount of current (power) used by the inverter when no load is active (lost power). The efficiency of the inverter is lowest when the load demand is low.

stand-off mounting — Technique for mounting a photovoltaic array on a sloped roof, which involves mounting the modules a short distance above the pitched roof and tilting them to the optimum angle.

starved electrolyte cell — A battery containing little or no free fluid electrolyte.

state-of-charge (SOC) — The available capacity remaining in the battery, expressed as a percentage of the rated capacity.

storage battery — A device capable of transforming energy from electric to chemical form and vice versa. The reactions are almost completely reversible. During discharge, chemical energy is converted to electric energy and is consumed in an external circuit or apparatus.

stratification — A condition that occurs when the acid concentration varies from top to bottom in the battery electrolyte. Periodic, controlled charging at voltages that produce gassing will mix the electrolyte. See also equalization.

string — A number of photovoltaic modules or panels interconnected electrically in series to produce the operating voltage required by the load.

sub-hourly energy markets — Electricity markets that operate on time steps of 5 minutes. Approximately 60% of all electricity in the United States is currently traded in sub-hourly markets, running at 5-minute intervals so that maximum flexibility can be obtained from the generation fleet.

substrate — The physical material upon which a photovoltaic cell is applied.

subsystem — Any one of several components in a photovoltaic system (i.e., array, controller, batteriesinverterload).

sulfation — A condition that afflicts unused and discharged batteries; large crystals of lead sulfate grow on the plate, instead of the usual tiny crystals, making the battery extremely difficult to recharge.

superconducting magnetic energy storage (SMES) — SMES technology uses the superconducting characteristics of low-temperature materials to produce intense magnetic fields to store energy. It has been proposed as a storage option to support large-scale use of photovoltaics as a means to smooth out fluctuations in power generation.

superconductivity — The abrupt and large increase in electrical conductivity exhibited by some metals as the temperature approaches absolute zero.

superstrate — The covering on the sunny side of a photovoltaic (PV) module, providing protection for the PV materials from impact and environmental degradation while allowing maximum transmission of the appropriate wavelengths of the solar spectrum.

surge capacity — The maximum power, usually 3-5 times the rated power, that can be provided over a short time.

system availability — The percentage of time (usually expressed in hours per year) when a photovoltaic system will be able to fully meet the load demand.

system operating voltage — The photovoltaic array output voltage under load. The system operating voltage is dependent on the load or batteries connected to the output terminals.

system storage — See battery capacity.


tare loss — Loss caused by a charge controller. One minus tare loss, expressed as a percentage, is equal to the controller efficiency.

temperature compensation — A circuit that adjusts the charge controller activation points depending on battery temperature. This feature is recommended if the battery temperature is expected to vary more than ±5°C from ambient temperature.

temperature factors — It is common for three elements in photovoltaic system sizing to have distinct temperature corrections: a factor used to decrease battery capacity at cold temperatures; a factor used to decrease PV module voltage at high temperatures; and a factor used to decrease the current carrying capability of wire at high temperatures.

thermophotovoltaic cell (TPV) — A device where sunlight concentrated onto a absorber heats it to a high temperature, and the thermal radiation emitted by the absorber is used as the energy source for a photovoltaic cell that is designed to maximize conversion efficiency at the wavelength of the thermal radiation.

thick-crystalline materials — Semiconductor material, typically measuring from 200-400 microns thick, that is cut from ingots or ribbons.

thin film — A layer of semiconductor material, such as copper indium diselenide or gallium arsenide, a few microns or less in thickness, used to make photovoltaic cells.

thin film photovoltaic module — A photovoltaic module constructed with sequential layers of thin film semiconductor materials. See also amorphous silicon.

tilt angle — The angle at which a photovoltaic array is set to face the sun relative to a horizontal position. The tilt angle can be set or adjusted to maximize seasonal or annual energy collection.

tin oxide — A wide band-gap semiconductor similar to indium oxide; used in heterojunction solar cells or to make a transparent conductive film, called NESA glass when deposited on glass.

total AC load demand — The sum of the alternating current loads. This value is important when selecting an inverter.

total harmonic distortion — The measure of closeness in shape between a waveform and it’s fundamental component.

total internal reflection — The trapping of light by refraction and reflection at critical angles inside a semiconductor device so that it cannot escape the device and must be eventually absorbed by the semiconductor.

tracking array — A photovoltaic (PV) array that follows the path of the sun to maximize the solar radiation incident on the PV surface. The two most common orientations are (1) one axis where the array tracks the sun east to west and (2) two-axis tracking where the array points directly at the sun at all times. Tracking arrays use both the direct and diffuse sunlight. Two-axis tracking arrays capture the maximum possible daily energy.

transformer — An electromagnetic device that changes the voltage of alternating current electricity.

transparent conducting oxide (TCO) — A doped metal oxide used to coat and improve the performance of optoelectronic devices such as photovoltaics and flat panel displays. Most TCO films are fabricated with polycrystalline or amorphous microstructures and are deposited on glass. The current industry-standard TCO is indium tin oxide. Indium is relatively rare and expensive, so research is ongoing to develop improved TCOs based on alternative materials.

tray cable (TC) – may be used for interconnecting balance-of-systems.

trickle charge — A charge at a low rate, balancing through self-discharge losses, to maintain a

tunneling — Quantum mechanical concept whereby an electron is found on the opposite side of an insulating barrier without having passed through or around the barrier.

cell or battery in a fully charged condition.

two-axis tracking — A photovoltaic array tracking system capable of rotating independently about two axes (e.g., vertical and horizontal).

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ultraviolet — Electromagnetic radiation in the wavelength range of 4 to 400 nanometers.

underground feeder (UF) — May be used for photovoltaic array wiring if sunlight resistant coating is specified; can be used for interconnecting balance-of-system components but not recommended for use within battery enclosures.

underground service entrance (USE) — May be used within battery enclosures and for interconnecting balance-of-systems.

uninterruptible power supply (UPS) — The designation of a power supply providing continuous uninterruptible service. The UPS will contain batteries.

utility-interactive inverter — An inverter that can function only when tied to the utility grid, and uses the prevailing line-voltage frequency on the utility line as a control parameter to ensure that the photovoltaic system’s output is fully synchronized with the utility power.


vacuum evaporation – The deposition of thin films of semiconductor material by the evaporation of elemental sources in a vacuum.

vacuum zero — The energy of an electron at rest in empty space; used as a reference level in energy band diagrams.

valence band — The highest energy band in a semiconductor that can be filled with electrons.

valence level energy/valence state — Energy content of an electron in orbit about an atomic nucleus. Also called bound state.

varistor — A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground.

vented cell — A battery designed with a vent mechanism to expel gases generated during charging.

vertical multijunction (VMJ) cell — A compound cell made of different semiconductor materials in layers, one above the other. Sunlight entering the top passes through successive cell barriers, each of which converts a separate portion of the spectrum into electricity, thus achieving greater total conversion efficiency of the incident light. Also called a multiple junction cell. See also multijunction device and split-spectrum cell.

volt (V) — A unit of electrical force equal to that amount of electromotive force that will cause a steady current of one ampere to flow through a resistance of one ohm.

voltage — The amount of electromotive force, measured in volts, that exists between two points.

voltage at maximum power (Vmp) — The voltage at which maximum power is available from a photovoltaic module.

voltage protection — Many inverters have sensing circuits that will disconnect the unit from the battery if input voltage limits are exceeded.

voltage regulation — This indicates the variability in the output voltage. Some loads will not tolerate voltage variations greater than a few percent.


wafer — A thin sheet of semiconductor (photovoltaic material) made by cutting it from a single crystal or ingot.

watt — The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage).

waveform — The shape of the phase power at a certain frequency and amplitude.

wet shelf life — The period of time that a charged battery, when filled with electrolyte, can remain unused before dropping below a specified level of performance.

window — A wide band gap material chosen for its transparency to light. Generally used as the top layer of a photovoltaic device, the window allows almost all of the light to reach the semiconductor layers beneath.

wire types — See Article 300 of National Electric Code for more information.

work function — The energy difference between the Fermi level and vacuum zero. The minimum amount of energy it takes to remove an electron from a substance into the vacuum.


zenith angle — the angle between the direction of interest (of the sun, for example) and the zenith (directly overhead).

Door Industry Terms & Definitions

Astragal- A molding or trim attached to the meeting edge of one door of a pair of door(s) which prevents swing-through and covers the gap where the doors meet when closed.

Balance Match- Two or more veneer components or leaves of equal size to make up a single face.

Barber Pole– An optical effect that often occurs when veneers are book matched, resulting in an alternating dark and light appearance. This is caused by slicing veneer, creating tight and loose sides that are placed adjacent to one another in the book matching process. The tight and loose sides reflect light differently, resulting in the varying appearance.

Bark Pocket- Bark around which normal wood has grown.

Bevel- A machine angle other than a right angle, i.e., a 3 degree bevel that is equivalent to a 1/8 inch drop in a 2 inch span (1 mm in 16mm).

Bird Peck- A mark or wound in a tree or piece of wood caused by birds pecking on the growing tree in search of insects.

Blended Repair Tapering- A repair referring to end splits, repaired with wood or filler similar in color to blend well with adjacent wood.

Blending- Color change that is detectable at a distance of 6 ft. to 8 ft. (1.8 m to 2.4 m) but which does not detract from the overall appearance of the door.

Blocking- A material used to replace core material in specific locations to provide improved screw holding for the attachment  of hardware or provide additional structural door reinforcement.

Bonded Core- Stiles and rails are securely glued to the core prior to application of crossbanding, door skins, veneers or laminate.

Book Match- Adjacent pieces of veneer from a flitch or log are opened like a book and spliced to make up the face with matching occurring at the spliced joints. The fibers of the wood, slanting in opposite directions in the adjacent sheets, create a characteristic light and dark effect when the surface is seen from an angle.

Bow- A curvature along the door height, or a deviation from a flat plane from end to end. A deviation, flatwise, from a straight line drawn from end to end or top to bottom of a piece as measured at the point of greatest distance from the straight line.

Burl- A figure created by abnormal growth or response to injury that forms an interwoven, contorted, or gnarly mass of dense woody tissue on the trunk or branch of the tree.

Burl, conspicuous- A swirl, twist or distortion in the grain of the wood which usually occurs near a knot or crotch. A conspicuous burl is associated with abrupt color variation and/or a cluster of small dark piths caused by a cluster of adventitious buds.

Butt Joint- A joint formed by square edge surfaces (ends, edges, and faces) coming together. Also referred to as a end butt joint, edge butt joint.

Cathedral Grain- A grain appearance characterized by a series of stacked and inverted “V”s, or cathedral type of springwood (earlywood) summerwood (latewood) patterns common in plain sliced (flat cut) veneer (see split heart).

Center Match- An even number of veneer components or leaves of equal size (prior to edge trimming) matched with a joint in the center of the panel to achieve horizontal symmetry.

Certified Wood- Wood products that have been qualified by an independent third party agency as satisfying their proprietary requirements for responsible environmental practices.

Checks- Small slits running parallel to grain wood, caused chiefly by strains produced in seasoning and drying.

Comb Grain- A rift cut veneer with exceptionally straight grain and closely-spaced growth increments resembling the appearance of long strands of combed hair

Compatible for color and grain- Components selected so that lighter-than-average color components are not adjacent to darker-than-average color components, no sharp contrasts in color exist between adjacent components, and grain of adjacent components does not vary widely and is similar in grain, character, and figure.

Compatible Species- Different species which are able to exist in a harmonious combination of color and grain.

Component (Of Face Veneer)- An individual piece of veneer that is joined to other pieces to achieve a full length and width face. Terms used interchangeably with component in the context of the face are piece and leaf.

Composite-  A composite whose ingredients include cellulosic elements. These cellulosic elements can appear in the form of, but are not limited to: distinct fibers, fiber bundles, particles, wafers, flakes, strands and veneers. These elements may be bonded together with naturally occurring or synthetic polymers. Also, additives such as wax or preservatives may be added to enhance performance.

Composite Panel- A panel composed of a wood derivative such as MDF. Used for opaque finishes.

Conspicuous- See burl, conspicuous and knots, conspicuous pin.

Core- The innermost layer or section in component construction. For typical constructions see:  Particleboard  Core, Medium Density Fiberboard Core, Structural Composite Lumber Core, Staved Lumber Core, Laminated Veneer Lumber Core, Fire Resistant Composite Core and other special core types.

Core, Fire Resistant- A door core material meeting fire listing requirements for fire protection of 20 minutes or greater.

Cross Bar (Veneer)- Irregularity of grain resembling a dip in the grain running at right angles, or nearly so, to the length of the veneer.

Cross Grain (Cross Figures)- Grain direction irregularity due to interlocked fibers, uneven annual rings, or to the intersection of a branch and stem forcing the visual line of the grain to run at an angle to the length of the wood. It is characterized by mild or dominant patterns across the grain in some faces. Also known as Cross Figures

Crossbanding- A ply placed between the core and face veneer in 5-ply construction or a ply placed between the back and face of a 3-ply skin in 7-ply construction, typically of hardwood veneer or engineered wood product.

Cup- A deviation in the face of a piece from a straight line drawn from end to end of a piece as measured at the point of greatest distance from the straight line.

Defect, Open- Checks, splits, open joints, knotholes, cracks, loose knots, wormholes, gaps, voids, or other opening interrupting the smooth continuity of the wood surface.

Discolorations- Stains in wood substances. Some common veneer stains are sap stains, blue stains, stain produced by chemical action caused by the iron in the cutting knife coming into contact with the tannic acid in the wood, and those  resulting from the chemical action of the glue.

Door Frame- A group of components (wood, composite, aluminum or steel) that are assembled to form an enclosure and support for a door. Also known as door jambs.

Door, Louver- A door assembly of stiles and rails where the interior is filled with slat or chevron louvers.

Doze- A form of incipient decay characterized by a dull and lifeless appearance of the wood, accompanied by a loss of strength and softening of the wood substance.

Edge Band- A strip along the outside edges of the two sides and/or top and bottom of the door (See stiles/vertical edges, rails/horizontal edges).

Edge Glued (Edge Joint)- When the edges of boards are glued together to increase the width.

End Match- Butting adjacent veneer leaves on one panel end to end in sequence. Veneer leaves are book matched end to end. Generally used for very long panels or for projects in which only short-length veneers are available.

Face Veneer- The outermost exposed wood veneer surface of a veneered wood door.

Few- A small number of characteristics (generally less than five) without regard to their arrangement in the panel.

Figure- The natural pattern produced in the wood surface by annual growth rings, rays, knots, and natural deviations. From the normal grain, such as interlock and wavy grain, and irregular coloration. 

Finger Joint- A series of interlocking fingers precision cut on the ends of two pieces of wood which mesh together and are held rigidly in place with adhesive.

Fire Rated Doors- A door complying with NFPA 80 that is listed and labeled by a qualified testing agency for fire-protection ratings. Ratings are indicated, based on testing at positive pressure, according to NFPA 252 or UL 10C, or at neutral pressure according to UL 10B. Doors can be rated as resisting fire for 20 minutes, 30 minutes, 45 minutes, 60 minutes, or 90 minutes. The door must be tested and carry an identifying label from a qualified testing and inspection agency.

Fire Resistant Composite Core- A core, typically incorporating minerals rather than wood fiber as the primary component, designed to improve fire resistance and thermal transmission,

Flake- See Fleck, Ray.

Flat-Cut- See Plain-Sliced.

Fleck, Ray (Flake)- Portion of a ray as it appears on the quartered or rift cut surface. Fleck is often the dominant appearance in oak.

Flitch-  A complete bundle of veneers sheets laid together in sequence as they are cut from a given log or section of a log.

Gaps- 1) An unfilled opening in a continuous surface or between adjoining surfaces. 2) An open slit in the inner ply or plies or improperly joined veneer when joined veneers are used for inner plies.

Grain- The direction, size, arrangement and appearance of the fibers in wood or veneer.

Grain Slope- Expression of the angle of the grain to the long edges of the veneer component.

Grain Sweep- Expression of the angle of the grain to the long edges of the veneer component over a 12 inch (300 mm) length from each end of the door.

Hairline- A thin, perceptible line showing at the joint of two pieces of wood.

Half-round Slicing- Veneer slicing method similar to rotary slicing, in which the piece being sliced is secured to a “stay log” device that permits the cutting of the log on a wider sweep than when mounted with its center secured in the lathe to produce rotary sliced veneer. Plain-sliced or flat-sliced veneer can be produced this way. A type of half-round cutting is used to achieve plain-sliced or flat-cut veneer

Hardboard- Homogeneous panels manufactured primarily from inter-felted lignocellulosic (wood) fibers consolidated under heat and pressure with density of 31 lb/ft3  (497 kg/m3) or more.

Hardwood- General term used to designate lumber or veneer produced from temperate zone deciduous or tropical broad- leaved trees in contrast to softwood, which is produced from trees which are usually needle bearing or coniferous. The term does not infer hardness in its physical sense.

Heartwood- The non-active center of a tree generally distinguishable from the outer portion (sapwood) by its darker color.

High Pressure Decorative Laminate (HPDL)- A high impact resistant surface material consisting of decorative surface paper impregnated with melamine resins pressed over multiple kraft paper layers saturated with phenolic resins, thermoset at high pressure and temperature.

High Density Fiberboard (HDF)- The generic name for a panel made out of exploded wood fibers that have been highly compressed and where the bonding of the wood fibers requires no additional materials, although resin is often added.

Holes, Worm- Holes resulting from infestation by worms greater than 1/16 inch (1.6 mm) in diameter and not exceeding 5/8 inch (16 mm) in length.

Intumescent- A material applied to the surface of flammable products to reduce flammability that expands when exposed to extreme heat or fire to fill any gap between the door and frame or between doors.

Joint- The common edge between two adjacent materials in the same plane.

Joint, Edge- Joint running parallel to the grain of the wood.

Joint, Open- Joint in which two adjacent pieces of veneer in the same plane do not fit tightly together.

Knife Cuts per inch (KCPI)- A measure of the smoothness of machined lumber. Can be determined by holding the surfaced board at an angle to a strong light source and counting the visible ridges per inch, usually perpendicular to the profile. The surface is smoother with more knife marks per inch.

Knife Marks- Very fine lines that appear across the panel veneer or wood solids that can look as though they are raised resulting from some defect in the lathe knife that cannot be removed with sanding.

Knot- Cross section of tree branch or limb with grain usually running at right angles to that of the piece of wood in which it occurs.

Knot Holes- Voids produced when knots drop from the wood in which they were originally embedded.

Knots, Blending Pin- Sound knots 6.4 mm (1/4 inch ) or less that generally do not contain dark centers. Blending pin knots are barely detectable at a distance of 1.8 m to 2.4 m. (6 ft. to 8 ft ), do not detract from the overall appearance of the panel, and are not prohibited from appearing in all grades.

Knots, Conspicuous Pin- Sound knots 6.4 mm (1/4 inch ) or less in diameter containing dark centers.

Knots, Open (Knot Holes)- Openings where a portion of the wood substance of the knot was dropped out, or where cross checks have occurred to produce an opening. (See Dead Knots)

Knots, Spike- Knots cut from 0o to 45o to the long axis of limbs.

Laminated Veneer Lumber Core (LVLC)- Manufactured by laminating veneer with all grain laid-up parallel. It can be manufactured by using various species of wood fiber in various thicknesses.

Lap (Veneer)- A condition where one piece of veneer in the same ply overlaps another piece.

Loose Side- In knife-cut veneer, that side of the sheet that was in contact with the knife as the veneer was being cut, and cutting checks (lathe checks) because of the bending of the wood at the knife edge.

Low Pressure Decorative Laminate (LPDL)- A decorative surface paper that is saturated with reactive resins. During hot press lamination, the resin flows into the surface of the substrate, creating a hard crosslinked thermosetting permanent bond and permanently changing the characteristics of both the paper and the board.

Louver- A panel constructed of slats installed in a door or openings to allowing various degrees of light, air, or sound passage. May be constructed as adjustable. Common types are slat and Chevron – an inverted “V” wood louver (vented or non-vented).

Medium Density Fiberboard (MDF)- A generic term for a composite panel product or core manufactured from lignocellulosic fibers and a bonding system cured under heat and pressure in a hot press by a process in which the added binder creates the entire bond. MDF is generally denser than plywood and particleboard

Medium Density Overlay (MDO)- Typically MDO is kraft paper saturated with resin and cured under high heat and pressure to make a hard, smooth, paintable surface.

Medium Density Fiberboard Core (MDFC)- Wood fiber and/or agri-fiber based materials that comply with ANSI A208.2.

Meeting Edges- Two adjacent door edges not separated by a mullion or transom bar. These are found in pair, Dutch door and door  & transom applications.

Mineral- See Streaks, Mineral.

Mineral Core- See Core, Fire Resistant.

Mineral Stain- Olive and greenish-black streaks believed to designate areas of abnormal concentration of mineral matter; common in hard maple, hickory, and basswood. Also called Mineral Streak.

Mineral Streaks- Sharply contrasting elongated discoloration of the wood substance.

Natural- When referring to color and matching, veneers containing any amount of sapwood and/or heartwood.

Neutral Pressure- A fire door test procedure where the neutral pressure plane is at or near the top of the door. Sometimes referred to as negative pressure.

Nominal- 1) A term that designates a stated dimension as being approximate and subject to allowances for variation. 2) The average sizes (width and thickness) of lumber just out of the sawmill before being processed into usable board stock. Always larger than finished dimensions.

Not Noticeable- Not readily visible without careful inspection (as a measurement of natural or machining characteristics).

Not Restricted- Allowed, unlimited.

Occasional- A small number of characteristics that arearranged somewhat diversely within the face.

Particleboard- A generic term for a composite panel primarily composed of cellulosic materials (usually wood), generally in the form of discrete pieces or particles, as distinguished from fibers, bonded together with a bonding system, and which may contain additives. Particleboard may be further defined by the method of pressing. When pressure is applied in the direction perpendicular to the faces as in a conventional multi platen hot press, they are defined as flat platen pressed. When the applied pressure is parallel to the faces, they are defined as extruded.

Particleboard Core- Wood fiber and/or agri-fiber based materials that comply with ANSI A208.1, minimum grade LD-1.

Pitch- An accumulation of resin that occurs in separations in the wood or in the wood cells themselves.

Pitch Pocket- A well-defined opening between the annual growth rings that contains pitch.

Pitch Streak- A well-defined accumulation of pitch in the wood cells in a more or less regular streak.

Pith- A small, soft core occurring in the center of the log.

Plain Slicing (Flat Cut)- Veneer sliced parallel to the pith of the log and approximately tangent to the growth rings to achieve flat cut veneer. Plain sliced veneer can be cut using either a horizontal or vertical slicing machine or by the half-round method using a rotary lathe.

Plank Matched- A face containing specially selected and assembled dissimilar (in color, grain and width) veneer strips of the same species, and sometimes grooved at the joints between strips, to simulate lumber planking.

Pleasing Match- A face containing components, which provide a pleasing overall appearance. The grain of the various components need not be matched at the joints. Sharp color contrasts at the joints of the components are not permitted.

Ply- A single sheet of veneer or several strips laid with adjoining edges that may or may not be glued, which forms one veneer lamina  in a glued panel. In some constructions, a ply is used to refer to other wood components such as particleboard or MDF.

Positive Pressure- A fire door test procedure where the neutral pressure plane is located at 40 inches (1 m) above the sill.

Quartered (Quarter-sliced, Quarter Cut)- unlimited straight grain appearance achieved through the process od quarter-slicing or through the use of veneer cut in any fashion that produces a straight grain effect. Cut is radial to the pith to the extent that ray fleck is produced, and the amount of fleck is not limited.

Rails/Horizontal Edges- Top and bottom edge bands of door.

Random Matched (Mismatched)- A panel having a face made up of  veneer strips of the same species which are selected and assembled without regard to color or grain, resulting in variations, contrasts and patterns of color and grain. Pleasing appearance is not required.

Ray Fleck- See Fleck, Ray.

Red/Brown- When referring to color and matching, veneers containing all heartwood, ranging in color from light to dark.

Repairs- A patch, shim, or filler material inserted and/or glued into veneer or a panel to achieve a sound surface.

Repairs, Blending- Wood or filler insertions similar in color to adjacent wood so as to blend well.

Rift Cut- A straight grain appearance achieved through the process of cutting at a slight angle to the radial on the half-round stay log or through the use of a veneer cut in any fashion that produces straight grain with minimal ray fleck.

Rotary Cut- Veneer produced by centering the entire log in a lathe and turning it against a broad cutting knife which is set into the log at an angle.

Rough Cut- Irregular shaped areas of generally uneven corrugation on the surface of veneer, differing from the surrounding smooth veneer and occurring as the veneer is cut by the lathe or slicer.

Running Match- The panel face is made from components running through the flitch consecutively. Any portion of a component left over from a face is used as the beginning component or leaf in starting the next panel.

Ruptured Grain- A break or breaks in the grain or between springwood and summerwood caused or aggravated by excessive pressure on the wood by seasoning, manufacturing, or natural processes. Ruptured grain appears as a single or series of distinct separations in the wood such as when springwood is crushed leaving the summerwood to separate in one or more growth increments.

Rustic- Lacking excessive refinement, having a rough surface or finish.

Sapwood- The living wood of lighter color occurring in the outer portion of a tree, sometimes referred to as sap.

Shake- A separation or rupture along the grain of wood in which the greater part occurs between the rings of annual growth. (See also RUPTURED GRAIN).

Sharp Contrast- For the purpose of this standard, this term means the veneer of lighter than average color should not be joined at  the edges with veneer of darker than average color, and that two adjacent pieces of veneer should not be widely dissimilar in grain, figure and natural character markings.

Show Through (Sanding)- A defect caused by excessive sanding such that the crossbanding is visible through the face veneer.

Show Through (Telegraphing)- See Telegraphing

Skin- The face layer, whether flat or contoured, of flush doors, stile-and-rail doors, bending laminations, etc., typically composed of hardwood plywood (usually 3 ply), hardboard, or composition pane.

Sliced- Veneer produced by thrusting a log or sawed flitch into a slicing machine, which shears off veneer in sheets.

Slight- Visible on observation, but does not interfere with the overall aesthetic appearance with consideration of the applicable grade and common species characteristics of the panel.

Slip Matching- A process in which a leaf from a flitch is slid across the leaf beneath and, without turning, spliced at the joint. This process results in a panel or door which consists of all either loose or tight sides, minimizing barber pole or metamerism.

Sound Transmission Class (STC)- A single number rating system derived from measured values of sound transmission loss or the acoustical performance of a building element, such as a door, window or wall. The higher the STC value, the better the rating and the better the acoustical performance value. Tested in accordance with ASTM E413 and E90.

Split Heart- A method of achieving an inverted “V” or cathedral type of springwood (earlywood)/summerwood (latewood), plain- sliced (flat-cut) figure by joining two veneer components of similar color and grain. A cathedral type figure must be achieved by a single component in AA grade; the split heart method is allowed in grades A and B. Each half of a split heart shall be subject to the minimum component width requirements for grade A and B faces.

Splits- Separations of wood fiber running parallel to the grain.

Splits, Hairline- A thin, perceptible line, separation, crack, or absence of wood fiber in the surface of a material or at the joint of two pieces of wood or veneer

Staved Lumber Core (SLC)- A door core made with any combination of blocks or strips of wood glued together.

Stiles/Vertical Edges- The upright or vertical pieces of the core assembly of a wood flush door. Measurement. The width of the vertical edge/stile is measured at its widest side (the wide side of a beveled door).

Streaks, Mineral- Sharply contrasting elongated discoloration of the wood substance.

Structural Composite Lumber Core (SCLC)- An engineered wood product that is made by fusing a network of wood strands together with a water-resistant adhesive to produce a strong, solid and stable product that has true structural properties with excellent screw holding properties and very high split resistance.

Sweep- See Grain

Tape- Strips of gummed paper or cloth sometimes placed across the grain of large veneer sheets to facilitate handling and sometimes used to hold the edges of the veneer together at the joints prior to gluing.

Telegraphing- In veneer or laminated work, a defect caused by outlines and/or surface irregularities, such as frame parts, core laps, voids, extraneous core matter, etc., that are visible through the face veneers, applied top veneer, or laminate sheet following pressing and/or finishing. Also known as Show Through.

Thermally Fused Decorative Laminate Panel- A polyester or melamine resin-impregnated paper, thermally fused under pressure to a composite core.

Tight Side- In knife-cut veneer, that side of the sheet that was farthest from the knife as the sheet was being cut and containing no cutting checks (lathe checks).

Transom- An operable or non-operable fenestration product that is designed to be a companion product installed above a door system.

Twist- 1) A distortion in a wood piece caused by freed tension in the grain as the wood was drying. The surface appears to twist as the four corners of any face are no longer in the same plane. 2) In passage doors, any distortion in the door itself and not its relationship to the frame or jamb in which it is hung, measured by placing a straight edge or a taut string on the concave face

Veneer (wood)- A thin layer or sheet of wood rotary-cut, sliced, or sawed from a log or flitch. Thickness may vary from, but not exceed, 0.3mm [.012”] to 6.4mm [.252”] thick.

Vine Marks- Bands of irregular grain running across or diagonally to the grain, which are caused by the growth of climbing vines.

Voids- See Gaps.

Warp- 1) Any deviation from a true or plane surface, including bow, crook, cup, twist, or any combination thereof. Warp restrictions are based on the average form of warp as it occurs normally. 2) In passage architectural wood doors, any distortion in the plane of a door itself and not its relationship to the frame or jamb in which it is to be hung. (See also BOW, CROOK, CUP, TWIST)

White- When referring to color and matching, veneers containing all sapwood, ranging in color from pink to yellow.

Wood Filler- An aggregate of resin and strands, shreds, or flour of wood, which is used to fill openings in wood and provide a smooth, durable surface.

Wood Flush Door- An assembly consisting of a core and one or more edgebands, with at least two plies of overlay on each side of the core assembly. All parts are composed of wood, wood derivatives, fire-resistant composites, or decorative laminates.

Wormholes- Holes resulting from infestation of worms.

Worm Track (Scar)- A natural mark caused by various types of wood-attacking larvae. Worm tracks often appear as sound discolorations running with or across the grain in straight to wavy streaks. Sometimes referred to as “pith flecks” in certain species of maple, birch, and other hardwoods because of a resemblance to the color of the pith.

ROOFING Industry Terms & Definitions

Attic: The open area above the ceiling and under the roof deck of a steep-sloped roof.

Asphalt Shingle: A shingle manufactured by coating a reinforcing material (felt or fibrous glass mat) with asphalt and having mineral granules on the side exposed to the weather.

Bundle: A package of shingles. There are 3, 4 or 5 bundles per square.

Caulk: Using mastic or asphalt cement to fill a joint to prevent leaks.

Closed Cut Valley:  A method of valley treatment in which shingles from one side of the valley extend across the valley while shingles from the other side are trimmed two inches from the valley centerline. The valley flashing is not exposed.

Collar: Also called a vent sleeve, a collar is a pre-formed flange placed over a vent pipe to seal the roof around the vent pipe opening.

Condensation: The change of water from vapor to liquid when warm, moisture-laden air comes in contact with a cold surface.

Counter Flashing: The portion of the flashing attached to a vertical surface to prevent water from migrating behind the base flashing.

Course: A row of shingles or roll roofing running the length of the roof.

Cricket: A peaked saddle construction at the back of a chimney to prevent accumulation of snow and ice and to deflect water around the chimney.

Cutout: The open portions of a strip shingle between the tabs.

Deck, Decking or Sheathing: The structural “skin” of a roof over which roofing is applied. Most new homes have decking made of plywood. There are four main types of decking commonly used on residential roofing projects:

  • Plywood: Plywood is strong, durable, and light. It comes in many grades with ratings from A to D. Use only exterior grade plywood for decking. The thickness of plywood depends on the spacing of the rafters.
  • OSB (Oriented Strand Board): OSB is cheaper than plywood, but not as strong as plywood, and does not hold nails as well as plywood. One side has a slip resistant coating and should be placed facing up.
  • Tongue and groove 2-by-6: If a roof will be seen from the inside (no ceiling installed), tongue and groove is used. It is a wood decking that provides great insulation without additional rigid roof insulation in moderate climates. Also, the boards can be painted or stained on the inside to match the interior.
  • Step sheathing: Step sheathing is used alone or in combinations with solid sheathing for installation of tiles or shakes. Step sheathing allows air circulations under the tiles by using 1-by-6 or 2-by-6 boards that are evenly spaced so that air can move under the tiles or shakes.

Dimensional Shingle: A shingle that is textured, overlayed, or laminated and designed to produce a three-dimensional effect. (Similar to Laminated shingle and Architectural shingle)

Dormer: A framed window unit that projects through the sloping plane of a roof.

Downspout: A pipe for draining water from roof gutters. A downspout is also called a leader.

Drip Edge: An L-shaped, non-corrosive, non-staining material (usually metal) used along the eaves and rakes to allow water run-off to drip clear of underlying construction.

Eave: The horizontal, lower edge of a sloped roof.

Edge Venting: The installation of a vent material along the roof edge (e.g., Starter Vent) as part of a ventilation system. Edge vent material should be used in conjunction with other venting material (e.g., ridge vent) as it not intended for use by itself.

Exposure: Portion of the shingle exposed to the weather. Exposure is measured from the butt of one shingle to the butt of the next.

Fascia or Fascia Board: (pronounced fey-shuh) Horizontal trim at the eaves that covers the rafter ends.

Felt: A flexible sheet that is saturated with asphalt and used as an underlayment. Sometimes called “tar paper”, the GAF brand used is called Shingle Mate, made with a fiberglass mat instead of a paper mat.

Felt: A flexible sheet that is saturated with asphalt and used as an underlayment. Sometimes called “tar paper”, the GAF brand used is called Shingle Mate, made with a fiberglass mat instead of a paper mat.

Flashing: Pieces of metal or roll roofing used to prevent seepage of water into a building around any intersection or projection in a roof such as vent pipes, chimneys, adjoining walls, dormers and valleys. Galvanized metal flashing should be minimum 26-gauge. There are 4 main types of flashing used in residential roofing systems

  • Valley flashing: This flashing is used in open valleys of the roof. Most often leaks are found in the valley flashings due to flashing that is nailed too tightly to the decking or shingles that are not trimmed far enough off the flashing.
  • Plumbing vent flashing: Plumbing vent flashing prevents rainwater from running into holes cut for pipes in the roof. This flashing is sold according to the size of the vent pipe and the roof angle. Roofing material is installed over the flashing.
  • Lead flashing: When working with tile roofs, lead flashing is used. In the case of a plumbing vent flashing, the lead flashing is actually molded to the shape of the tile’s surface. Then the top of the lead flashing is covered by the next tile to prevent water from seeping under the flashing.
  • Step flashing: When a chimney or dormer wall intercepts the slope of the roof, step flashing is used. Step flashing is usually a metal piece that is bent in the middle, so that one end lays on the roof, and the other against the vertical wall of the dormer or chimney.

Gable: The upper portion of a sidewall that comes to a triangular point at the ridge of a sloping roof.

Gable Roof: A type of roof containing sloping planes of the same pitch on each side of the ridge. A gable roof typically contains a gable at each end.

Gambrel: A type of roof containing two sloping planes of different pitch on each side of the ridge. The lower plane has a steeper slope than the upper. A gambrel roof usually contains a gable at each end, just like a standard gable roof.

Granules: Ceramic-coated colored crushed rock that is applied to the exposed surface of asphalt roofing products.

Gutter: The trough that channels water from the eaves to the downspouts.

Hip: The inclined external angle formed by the intersection of two sloping roof planes. The hip runs from the ridge to the eaves.

Hip Roof: A type of roof containing sloping planes of the same pitch on each of four sides. A hip roof contains no gables.

Ice Dam: Ice dams occur when snow melts near the ridgelines of warm roofs (roofs without adequate ventilation). As the water runs down the roof to the overhang, it cools and freezes. If the snow continues this melt and freeze process, an ice dam can form that can seep under the shingles, through the decking and into the house. This, of course, can cause serious roof leaks–even in freezing temperatures. The best prevention to ice dams is a well-ventilated (cool) roof. Additional protection for your roof can be applied with an impermeable ice and water membrane. The membrane is installed on top of the decking, under the roofing material. Temporary prevention of ice dams can also be done through the use of electric cables along the eaves of the roof (where the dams usually form). However, new ice dams can form above the cables and still cause extensive damage. Another emergency solution to ice dams is to fill a sock or nylon with calcium chloride. Lay the stocking vertically across the ice dam. The calcium chloride will melt the ice and release the water so that it can drain outside, and not inside your roof.

Ice Guard: A self-adhering, waterproofing shingle underlayment designed to protect against water infiltration due to ice dams or wind-driven rain. GAF brand used would be called Weather Watch.

Intake Ventilation: The part of a ventilation system used to draw fresh air in. Usually vents installed in the soffit or along the eaves of a building.

Intake Ventilation: The part of a ventilation system used to draw fresh air in. Usually vents installed in the soffit or along the eaves of a building.

Laminated Shingles: Strip shingles containing more than one layer of tabs to create extra thickness. Laminated shingles are also called three-dimensional shingles.

Mansard Roof: A type of roof containing two sloping planes of different pitch on each of four sides. The lower plane has a much steeper pitch than the upper, often approaching vertical. (has no gables)

Open Valley: Method of valley construction in which shingles on both sides of the valley are trimmed along a chalk line snapped on each side of the valley. Shingles do not extend across the valley. Valley flashing is exposed.

Overhang: That portion of the roof structure that extends beyond the exterior walls of a building.

Pitch: Also known as “slope”, pitch is the measure of how “steep” a roof is. For example, if a roof is “4 in 12″, the roof rises 4 inches for every horizontal run of 12 inches. The pitch of the roof is a big factor in determining the kinds of materials that can be used and the longevity of the roof. Usually, a steeper roof (higher pitch) will last longer due to its better drainage capabilities.

Racking: Roofing application method in which shingle courses are applied vertically up the roof rather than across and up. Not a recommended procedure.

Rafter: The supporting framing member immediately beneath the deck, sloping from the ridge to the wall plate.

Rake: The inclined edge of a sloped roof over a wall from the eave to the ridge.

Re-roofing: Installing a new roof system on a building that is not new.

Ridge: The uppermost, horizontal external angle formed by the intersection of two sloping roof planes.

Ridge Vent: Vent placed along the ridge of the roof. It allows ventilation of the roof by raising the LEVEL of the ridge slightly leaving room for air flow. A filtration fabric placed in the side vents allows air to move through while preventing insects from entering.

Rise: The vertical distance from the eaves line to the ridge.

Run: The horizontal distance from the eaves to a point directly under the ridge.

Sheathing: Exterior grade boards used as a roof deck material. “Step sheathing” is used alone or in combinations with solid sheathing for installation of tiles or shakes. Step sheathing allows air circulations under the tiles by using 1-by-6 or 2-by-6 boards that are evenly spaced so that air can move under the tiles or shakes.

Shed Roof: A roof containing only one sloping plane. It doesn’t have hips, ridges, valleys or gables.

Slope: The degree of roof incline expressed as the ratio of the rise, in inches, to the run, in feet.

Soffit: The finished underside of the eaves.

Soil boot: A vent pipe that penetrates the roof.

Span: The horizontal distance from eaves to eaves.

Square: A unit of roof measure covering 100 square feet (10 feet by 10 feet).

Starter Strip: Asphalt roofing applied at the eaves that provide protection by filling in the spaces under the cutouts and joints of the first course of shingles.

Steep Slope Application: Method of installing asphalt shingles on roof slopes greater than 21 inches per foot.

Step Flashing: Flashing application method used where a vertical surface meets a sloping roof plane.

Tab: The exposed portion of strip shingles defined by cutouts.

Three-dimensional Shingles: See laminated shingles.

Three-tab Shingle: The most popular type of asphalt shingle usually 12″ x 36″ in size with three tabs.

Underlayment: A layer of asphalt saturated (sometimes referred to as tar paper) which is laid down on a bare deck before shingles are installed to provide additional protection for the deck.

Valley: The internal angle formed by the intersection of two sloping roof planes to provide water runoff.

Vent: Any outlet for air that protrudes through the roof deck such as a pipe or stack. Any device installed on the roof, gable or soffit for the purpose of ventilating the underside of the roof deck.

Woven Valley: Method of valley construction in which shingles from both sides of the valley extend across the valley and are woven together by overlapping alternate courses as they are applied. The valley flashing is not exposed.

GUTTER Industry Terms & Definitions

  • Downspout

    A pipe that carries rainwater from a roof gutter down to a drain or to the ground.

  • Drainage

    A system of gutters and drainpipes that carry water away from the foundation of a house.

  • Drip-Edge Extension (Also Called Back Flashing)

    Prevents water from washing over the fascia and eventually causing it to rot. If your fascia boards have black streaks on them, that usually means that the drip-edge is failing and the contractor may suggest adding an extension for an additional charge.

  • Drop Outlet

    A formed piece that serves as the hole from which the water travels from the horizontal section of the gutter to the downspout.

  • Elbow

    Pre-finished angled piece for directing water flow.

  • End Cap

    Flat, formed piece that is placed at the end of a gutter section.

  • Fascia Angle

    If there is any fascia board that is angled to follow the roof rafters, there is usually an extra charge.

  • Fascia Board

    A flat board that runs horizontally along the eaves of a roof, typically capping the ends of the roof rafters to give the roof edge a more finished look and provide a base for attaching gutters.

  • Fascia Replacement

    New gutters need a solid fascia surface upon which to be mounted. If rotted boards need to be replaced, there is a charge per linear foot. Fascia boards narrower than 3″ in width must be built out to provide proper support for hanging your new gutters and require an additional charge per linear foot.

  • Gauge

    The thickness or the gauge of the gutter is very important. Primary aluminum is the thickest and most consistent available. The gauges range from .019″ to .032″. The optimal thickness is .032″, so when buying aluminum gutters, insist on primary aluminum, which is the best.

  • Gutters

    Horizontal channels installed at the edge of a roof to carry rainwater or melted snow away from the house.

  • Hanger

    Metal bracket installed on fascia to hold up a horizontal section of gutter.

  • Inside Mitre Box

    A corner piece of the horizontal section that is deflected in.

  • Leader

    A pipe that carries rainwater from the gutters to the ground, sewers, or wells.

  • Mitres (Corners)

    When a company fabricates the corners on-site using the same material as the rest of the house, the advantage is that there is one seam instead of three and it’s a cleaner look overall.

  • Outside Mitre Box

    A corner piece of the horizontal section that is deflected out.

  • Pitch

    The angle at which a horizontal section of gutter is tilted in order to force water to flow toward a downspout.

  • Run Height

    The height (in stories) at which each gutter run will be located (first story, second story, etc.). This is used to determine the length of downspout needed.

  • Run Length

    A straight section of gutter mounted against the fascia board. It’s measured in linear feet, which measures the same as regular feet.

  • Seamless Gutter Machine

    The chosen gutter material is rolled through the forming machine to create a continuous piece of gutter that is custom-sized to fit your home.

  • Splash Block

    Plastic or concrete surface put under a downspout to direct water away from the house.

  • Strap

    Flat hangers that are nailed into the house to hold the downspouts in place.

SIDING Industry Terms & Definitions

Battens: Thin strips of wood that seal the joints of vertical wood planking. 

Backerboard: A flat material used on the face of the house, between the studs and the siding, to provide a nailable surface for the siding.

Beveled Clapboards: Tapered clapboards rather than a perfectly cut rectangular. 

Blind Nail: An installation for Hardie siding where each board is nailed corner to corner, on a nailing line, and then overlapping that nailed layer with the next board—so that all siding nails are hidden.

Blow-In Insulation: Blowing or spraying loose insulation product into cavities, attics, and floors for better R- value, home heating efficiency.

Board and Batten: A style in which a narrow strip of siding appears to cover the seam between two wider boards. Board and batten siding is installed vertically.

Brick veneer: A process of constructing a wall which involves a layer of bricks being attached to a wood framework of a house using brick ties. 

Butt Seam /Joint:  The gap between the exposed ends of lap siding.

Capillary Break: A hydrophobic material uses in the gap between parallel layers of siding and roofing.

Caulking: A waterproof filler or sealant used to seal joints. 

Channel: The area of the accessory trim or corner post where siding or soffit panels are inserted. Channels also refer to the trim itself, and are named for the letters of the alphabet they resemble, for example J-channel and F-channel are available.

Checking: A split or crack that shows up in the wood grain of a plank causing the plank to cup or bow.

Clapboard: Overlapping, horizontal wood plank siding made from either rectangular planks or taped planks.

Color fading: A common occurrence in vinyl and painted wood siding when vulnerable to the elements, expansion and contraction, as well as sun rays.

Color Plus: Hardie’s factory application of siding paint which provides extra protection against the elements, bugs, water, fire, and wood peckers. It is beautiful, baked-on, long-lasting, and vibrant.

Contemporary: A common term used for “modern” siding styles such as the popular Board and Batten look.

Course: A row of panels, one panel wide, running the length of the house from one side to the other or, in the case of vertical siding, from top to bottom.

Crown Moulding: A form of cornice created out of decorative molding installed along rooflines and window & door openings.

Cupping: A warp across the board in wood plank siding.

Dormer: A raised section of the roof. Dormers commonly contain a window that projects vertically through the slope in the roof.

Drip Cap/Head Flashing: An accessory installed with vertical siding to ensure that water drips away from panels and does not infiltrate them; it is also used as a vertical base.

Eaves: Eaves are the overhang of the roof.

Face Nailing: This is the act of driving a nail through the part of the panel that you can see.

Fascia Board: A board attached to the ends of the rafters between the roofing material and the soffit overhang. Fascia cap is the covering around that board.

Fiberboard Siding: is a building material used to cover the exterior of a building in both commercial and domestic applications. 

Fiber cement is a composite material made of cement reinforced with cellulose fibers.

WINDOWS Industry Terms & Definitions

Adjustable — accessible without major reconstruction of the window, door, TDD, SSP, roof window, or unit skylight to bring the parts of the product to a true or more effective relative position.

Air leakage — the flow of air that passes through fenestration products.

Airspace — the space between adjacent layers in a multi-layer glazing system.

Architectural terrace door — a door primarily used for terrace access in high-rise applications/buildings.
Note: Architectural terrace doors consist of one or more glazed panels contained within one master frame. The operable panels will be hinged on either jamb and can swing either to the exterior or interior (not both). The door is not used as a primary entrance door because of the nature of the sill/threshold design used to meet performance requirements.

Awning, hopper, projected window — a window consisting of one or more sash hinged at the top or bottom which project outward or inward from the plane of the frame. An awning rotates about its top hinge(s) and projects outward. A hopper window rotates about its bottom hinge(s) and projects inward.

Awning window — see Awning, hopper, projected window.

Balance — a mechanical device used in hung windows as a means of counterbalancing the weight of the sash.

Basement window — any window type intended for ventilating or illuminating a basement or cellar.

Bite — the dimension by which the inner or outer edge of the frame or glazing stop overlaps the edge of the glazing.

Brickmold — a molding used as an exterior door or window casing.

British thermal unit (Btu) — the heat required to increase the temperature of 1 lb. of water 1°F.

Building envelope — the assembly or assemblies of materials and components that enclose building spaces and are exposed to exterior space or separate conditioned interior space from unconditioned interior space.

Casement window — a window consisting of one or more sash hinged to open from the side (adjacent to the jambs), which project outward or inward from the plane of the frame in the vertical plane.

Cellulosic composite material — a composite material whose ingredients include cellulosic elements.
Note: These cellulosic elements appear in the form of, but are not limited to, distinct fibers, fiber bundles, particles, wafers, flakes, strands, and veneers.

Certification — a process that indicates a representative sample of a product line has been tested, that the product meets specified requirements, and that the product is subject to ongoing inspections by an outside certification agency.

Check rail — see Meeting rail.

Chemically bonded (when related to a welded corner) — a process where the two polymer profiles or pieces are heated and fused together with the aid of a chemical reaction. The reaction and bonding is similar to the original extrusion process.

Cladding — see Fenestration cladding.

Closing force — see Operating force and Force to latch door.

Combination assembly — an assembly formed by a combination of two or more separate fenestration products whose frames are mulled together utilizing a combination mullion or reinforcing mullion.

Commercial entrance system — a system of products used for ingress, egress, and rescue generally in non-residential buildings.
Note: Commercial entrance systems typically utilize panic hardware, automatic closers, and relatively large amounts of glass. Commercial entrance systems are often site assembled. They are typically subject to high use and possibly abuse and are designed to withstand such use and abuse.

Composite unit — a fenestration product consisting of two or more sash, leaves, lites, or sliding door panels within a single frame utilizing an integral mullion.
Note: Composite units are not to be confused with products made from cellulosic composite materials.

Concentrated load — a force applied to a fixed point on a window, door, TDD, SSP, roof window, or unit skylight component.

Condensation — the deposition of moisture (liquid water or frost) on the surface of an object caused by warm, moist air coming into contact with a colder object.

Conditioned space — an area or room within a building that (a) is heated or cooled by any equipment or appliance; (b) contains uninsulated ducts; or (c) has a fixed opening directly into an adjacent area or room that is heated or cooled by any equipment or appliance or contains uninsulated ducts.

Corrosion — the deterioration of a material by chemical or electrochemical reaction resulting from exposure to weathering, moisture, chemicals, or other agents or media.

Curtain wall — a non-load-bearing exterior wall cladding that is hung to the exterior of the building, usually spanning from floor to floor.
Note: Curtain wall systems can be factory-glazed or designed to accommodate field fabrication and glazing, including optional structural glazing. Curtain wall employs deep rectilinear framing profiles (approximately 150 mm [6 in] or greater), which are often made available in “stock lengths”. Curtain wall vertical framing members run past the face of floor slabs, and provision for anchorage is typically made at vertical framing members only. In contrast to combination assemblies and composite units, non-residential curtain wall systems often need to meet additional performance requirements for interstory differential movement, seismic drift, dynamic water infiltration, etc. Operating vents and entrance doors are provided as separate inserts.

Deflection — displacement due to flexure of a member under an applied load.

Design pressure (DP) — a rating that identifies the load, induced by wind and/or static snow, that a product is rated to withstand in its end-use application.
Note: Design pressure (DP) is not to be confused with Performance Grade (PG) or structural test pressure (STP). Loads induced by static snow are applicable only to TDDs, roof windows, and unit skylights.

Design wind load — the wind load pressure a product is required by the specifier to withstand in its end-use application.
Note: When other loads such as snow load are included, a “design load” results.

Divider — a member that divides glazing into separate vision areas. Dividers are either structural or decorative. Other common terms are muntin, true divided lite (TDL), simulated divided lite (SDL), grill, grid, or bar-in-glass.

Door — a means of access for the purpose of ingress and egress. See also Commercial entrance system, Dual-action side-hinged door, Folding door system, Interior door, Passive door, Revolving door, Secondary storm product, Side-hinged door system, Sliding door, Storm door, and Vehicular-access door.

Double-hung window — a hung window with two sash in which both sash are operable.

Dual-action side-hinged door — a door system consisting of one or more leaves contained within an overall frame and designed such that one of the leaves is operable in a swing mode and can be tilted inward from the top for ventilation.

Dual-action window — a window consisting of a sash that tilts from the top and swings inward from the side for cleaning of the outside surface. Also referred to as a tilt-turn window.

Dual glazing — two layers of glazing material mounted in a common frame and/or sash, separated by a space, and sealed or non-sealed.

Dual mode — the primary and secondary window/door, or both primary windows/doors, are closed, the primary windows/doors are locked, and the insect screen (when offered or specified by the manufacturer) is in the stored position.

Fenestration — openings in the building envelope, such as windows, doors, secondary storm products (SSPs) curtain walls, storefronts, roof windows, tubular daylighting devices (TDDs), sloped glazing, and skylights, designed to permit the passage of air, light, or people.

Fenestration cladding — the exterior components that cover the frame, sash, leaf, or sliding door panel members and constitute the weather-resistant surface.
Note: Some claddings function only as an aesthetic covering, while others contribute partially to the structural strength of the product. This use of cladding should not be confused with the definition of “Components and Cladding — Elements of the building envelope that do not qualify as part of the main wind-force resisting system” found in ASCE/SEI 7.

Fixed door — one or more non-operable assembled leaves or sliding door panels within a door frame and threshold/sill.

Fixed window — a window that is designed to be non-operable and consists of a glazed frame or a non-operating sash within a frame.
Note: This category does not include non-operable unit skylights or TDDs, or products fabricated from curtain wall or storefront systems that are used in window openings.

Float glass — flat glass that has been formed on molten metal, commonly tin. The surface in contact with the tin is known as the tin surface or tin side. The top surface is known as the atmosphere side or air side.

Folding door system — a door system that has, at a minimum, a hinge or pivot attachment of any type between two leaves and three vertical axes about which the leaves rotate. The leaves can be folded to the interior or exterior of the opening. These systems are either top hung or bottom supported by hardware that attaches to a single track system and include, at a minimum, two pivoting/folding leaves, a frame, and a track and roller assembly. The frame has vertical and horizontal members that are joined at the intersections that fully encompass the operating and inactive leaves in a closed position. A flush set track assembly can exist in place of a sill assembly. Additional hinged and pivoting/folding leaves and/or a single-side hinged leaf can be included in the door system.

Forced-entry resistance (FER) — the ability of a window or door in the locked position to resist entry under a specified load and conditions.

Force to latch door — the force required to close a door and fully engage the latch.

Frame — the enclosing structure of a window, door, TDD, roof window, SSP, or unit skylight which fits into or attaches to the wall or roof opening and receives glazing, sash, panels, leaves, or vents.

Fully tempered glass — glass that has been heat treated to a high surface and/or edge compression to meet the requirements of ASTM C1048 (kind FT) or CAN/CGSB 12.1.
Note: Fully tempered glass, if broken, will fracture into many small pieces (dice) which are more or less cubical. Fully tempered glass is approximately four times stronger than annealed glass of the same thickness when exposed to uniform static pressure loads.

Fusion welded — see Welded.

Garage door — see Vehicular-access door.

Garden window — see Greenhouse window.

Gateway performance requirements — the requirements for minimum gateway test size, air leakage resistance, structural design load and overload testing, water penetration testing, forced-entry resistance, and auxiliary testing which are the conditions permitting a product entry into a Performance Class.

Gateway test size — the minimum test specimen size specified to enter a Performance Class.

Glass — a hard, brittle substance, usually transparent, made by fusing materials such as soda ash (NA2CO3), limestone (CaCO3), and sand under high temperatures.

Glazing — (n): an infill material such as glass or plastic. (v): the process of installing an infill material into a prepared opening in windows, doors, TDDs, roof windows, SSPs, or unit skylights.

Grade — see Performance Grade (PG).

Greenhouse window (garden window) — a window consisting of a three-dimensional, five-sided structure, with provisions made for supporting plants in the enclosed space outside the plane of the wall. Operating sash are allowed but are not required.

Handle — a component which enables the movement of a sash, leaf, or panel, or which activates a mechanism which locks or unlocks a sash, leaf, or panel.

Hardware — all the necessary equipment to retain, operate, and lock or unlock the sash, leaf, or panel within the frame.

Head — the horizontal member forming the top of the frame.

Heat-strengthened glass — glass that has been heat treated to a specific surface and/or edge compression range to meet the requirements of ASTM C1048 (kind HS).
Note: Heat-strengthened glass is approximately two times as strong as annealed glass of the same thickness when exposed to uniform static pressure loads. Heat-strengthened glass is not considered safety glass and will not completely fracture into many small pieces (dice) as with fully tempered glass.

Heat treated — see Fully tempered glass and Heat-strengthened glass.

Hinged rescue window — any window that is mounted into a stationary perimeter frame and is permanently hinged at one jamb.

Hopper window — see Awning, hopper, projected window.

Horizontally pivoted window — see Pivoted window.

Horizontal sliding window — a window that consists of one or more sash that slide or roll horizontally within a common frame and can also contain fixed lites/sash.
Note: Typically, operating sash are identified with an (X) and fixed lites or fixed sash are identified with an (O).

Hung window — a window consisting of vertically sliding sash which utilize counterbalancing devices to allow the sash to be opened to any variable position between its fully open and fully closed limits. See also Vertical sliding window.
Note: Common types are single hung, double hung, and triple hung.

Inoperable — no longer opening, closing, locking, or unlocking as originally designed.

Insulating glass unit (IG unit or IGU) — two or more lites of glass spaced apart and hermetically sealed to form a single unit with an air- or gas-filled space between each lite.

Integral ventilating system/device — an apparatus that is independent from but installed into a window, door, or unit skylight product for the purpose of controlling the transfer of air through the window, door, or unit skylight product.

Interior accessory window (IAW) — a glazed frame and/or sash, attached inboard of existing prime windows, curtain wall, or storefront, in commercial buildings, to enhance control of thermal transmittance, solar heat gain, sound, air leakage, and/or daylight. IAWs are not intended for occupant operation or to be used with the exterior windows in the open position, nor are they intended to provide any specific resistance to air leakage or water penetration, or withstand structural load.
Note: The IAW frame is typically anchored to surrounding construction, to the existing window frame, or to the curtain wall or storefront interior frame. Unlike SSPs and multiple glazing panels, interior accessory windows are intended for use by trained custodial personnel only and are fitted with limited-access custodial locks to hinge or lift out for periodic cleaning of the non-hermetically sealed air space created. If IAWs are intended for regular occupant operation, or used with the exterior windows in the open position, the product should instead be rated as a prime window or SSP.

Interior door — a door system not intended for use in exterior applications.

Interior window — a window system not intended for use in exterior applications.

Jal-awning window — a window consisting of a multiplicity of top-hinged sash arranged in a vertical series within a common frame, each operated by its own control device which swings the bottom edges of the sash outward. See also Jalousie window and Tropical awning window.

Jalousie window — a window consisting of a series of overlapping horizontal frameless louvers which pivot simultaneously in a common frame and are actuated by one or more operating devices so that the bottom edge of each louver swings outward and the top edge swings inward during operation.

Jambs — the upright or vertical members forming the side of the frame.

Laminated glass — two or more lites of glass permanently bonded together with one or more polymer interlayers.

Leaf — a part of a side-hinged door system, glazed or unglazed, surrounded by a frame. Leaves can be fixed in place (non-operable) or movable (operable).

Limited water (LW) (as a designation) — that the water penetration resistance performance is achieved by testing at a pressure less than the minimum test pressure required for the indicated Performance Class and Performance Grade (PG).

Lite (light) — a pane of glass or an insulating glass (IG) unit used in a window, door, TDD, roof window, SSP, or unit skylight. Frequently spelled “lite” in industry literature to avoid confusion with visible light.

Manufacturer — a company which fabricates and/or assembles one or more parts, components, and/or accessories or supplies entire fenestration systems.

Meeting rail or check rail — one of the two adjacent horizontal sash members that come together when in the closed position.

Meeting stile — one of the two adjacent vertical leaf, sash, or panel members that come together when in the closed position.

Minimum gateway test size — the test specimen size specified to enter a Performance Class at the lowest or minimum level.

Moisture content — the percentage of dry weight that is composed of water, such as in wood.

Mullion assembly

Combination mullion — a horizontal or vertical member formed by joining two or more individual fenestration units together without a mullion stiffener.

Integral mullion — a horizontal or vertical member which is bounded at either end or both ends by crossing frame members.

Mullion stiffener — an additional reinforcing member used in a reinforcing mullion. Mullion stiffeners carry the entire load or share the load with adjacent framing members.

Reinforcing mullion — a horizontal or vertical member with an added continuous mullion stiffener and joining two or more individual fenestration units along the sides of the mullion stiffener.

Multiple glazing panel (MGP) — a glazed panel that can be installed in or on a sash, leaf, or panel on either the interior side or exterior side of the primary glazing. An MGP is tested only in conjunction with a specific primary window or door.

Muntin — see Divider.

Negative pressure — pressure acting in the outward direction.

Non-hung window — a window consisting of vertically sliding sash which utilize mechanical retainers or slide bolts to allow the sash to be opened to any one of the pre-selected positions between its fully open and fully closed limits. See also Vertical sliding window.

Non-operable — intended to not open or close.

Normal use (pertaining to windows, doors, secondary storm products, operable unit skylights, and roof windows) — intended for operation for reasons in addition to cleaning and maintenance of the window(s), door(s), secondary storm product(s), operable unit skylight(s), or roof window(s) in question.

Operable — intended to be opened and closed.

Operating force — the force required to initiate or maintain a sash, leaf, or panel motion in either the opening or closing direction.

Outdoor-indoor transmission class (OITC) — a single-number rating calculated in accordance with ASTM E1332, using values of outdoor-indoor transmission loss, that provides an estimate of the sound insulation performance of a facade or building elements.
Note: The frequency range used is typical of outdoor traffic noises.

Overall dimensions — the external height and width of the product, expressed in millimeters or inches.

Panel — the members of a sliding door or sliding door side lite within a frame which are designed to accommodate the glazing.

Parallel opening window — a window consisting of an operable sash that moves outward in a horizontal direction perpendicular to the plane of the frame for the purpose of ventilation. The sash remains parallel to the frame throughout its range of motion.

Passive door — one or more hinged leaves or sliding door panels that are normally held inactive by latching or locking hardware, but can become active on the release of the latching or locking hardware.

Performance Class — one of the five Performance Classes (R, LC, CW, AW, and SK) within the classification system that provides for several levels of performance.
Note: This allows the purchaser or specifier to select the appropriate level of performance depending on climatic conditions, height of installation, type of building, etc.

Performance Grade (Grade or PG) — a numeric designator that defines the performance of a product in accordance with this Standard/Specification.
Note: Performance Grade (Grade or PG) is not to be confused with design pressure (DP) or structural test pressure (STP). Performance Grade (PG) is achieved only on successful completion of all applicable tests specified

Pivot — an axis or the hardware about which a window, sash, panel, or leaf rotates.

Pivoted window — a window consisting of a sash which pivots about an axis within the frame. The pivoting action of the window allows for easy access to clean the outside surfaces of the window. Two common types are the 180° compression seal pivoting window and the 360° pivoting window.

Plastic glazing — plastic infill materials (including, but not limited to, acrylic, co-polyester, fiberglass-reinforced plastic, and polycarbonate) that are glazed or set in a frame or sash.

Positive pressure — pressure acting in the inward direction.

Primary door — that door in a dual-door system so designated by the manufacturer, capable of protecting the building’s interior from climatic elements (as opposed to a secondary door used mainly for performance enhancement).

Primary window — that window in a dual-window unit so designated by the manufacturer, capable of protecting the building’s interior from climatic elements (as opposed to a secondary window used mainly for performance enhancement).

Rail — a horizontal member of a sash, leaf, or panel.

Reinforcement — the material added to individual sash, leaf, panel, or frame members to increase strength and/or stiffness.

Revolving door — an exterior door consisting of two or more leaves that pivot about a common vertical axis within a cylindrically shaped vestibule.

Roof window — a sloped application of a fenestration product that provides for in-reach operation.
Note: Roof windows used for emergency escape and rescue usually have a balanced sash.

Rough opening — the opening in a wall or roof into which a window, door, TDD, roof window, or unit skylight is to be installed.

Safety glass — a strengthened or reinforced glass that is less subject to breakage or splintering, such as glass for doors, unit skylights, and some windows. See also Fully tempered glass and Laminated glass.

Sash — the members of a window, secondary storm product, or unit skylight that fit within a frame which are designed to accommodate the glazing.

Screen — a product that is used with a window, door, secondary storm product, or unit skylight, consists of a mesh of wire or plastic material used to keep out insects, and is not for providing security or retention of objects or persons from the interior.

Sealant — a compound used to fill and seal a joint or opening.

Secondary door — that door in a dual-door system so designated by the manufacturer, used on the exterior of, or interior of, and in tandem with, a primary door designated by the manufacturer to be used for the purpose of performance enhancement, and not to be used by itself as a primary door.

Secondary storm product (SSP) — a door, window, or skylight product intended to be used only in conjunction with a primary door, window, or skylight product for the purpose of enhancement of performance in a system with the primary product. A secondary storm product can be attached to the internal or external frame or sash of the primary product. A secondary storm product is also considered a secondary door or window.

Secondary window — that window in a dual-window unit so designated by the manufacturer, used on the exterior of, or interior of, and in tandem with, a primary window for the purpose of performance enhancement, and not to be used by itself as a primary window.

Serviceable — accessible without major reconstruction of the window, door, SSP, TDD, roof window, or unit skylight.

Setting block — a device or member that supports the weight of the glazing and is in direct contact with an edge of the glazing after final installation.

Side-hinged door system — a door system having, at a minimum, a hinge attachment of any type between a leaf and jamb, mullion, or edge of another leaf but having a single, fixed vertical axis about which the leaf rotates between open and closed positions. These systems include, at a minimum, a single operating leaf, surrounding frame, and components. The surrounding frame has vertical and horizontal members that are joined at the intersections and fully encompass the operating and/or fixed leaf/leaves.
Note: Additional operating, passive and/or fixed leaves, side lites, transoms, framing, and mullions are often included.

Side-hinged (inswinging) window — a window that consists of sash hinged at the jambs that swings inward using exposed butt hinges or concealed butt hinges, and in some cases friction hinges. It is used primarily for cleaning or emergency escape and rescue purposes, but not for ventilation other than in the case of emergency. The gateway test size is larger than for casement windows, but otherwise the same requirements are met. See also Hinged rescue window and Top-hinged window.

Side lite — an operable or non-operable product that is designed to be a companion product installed on one or both sides of an operable door or a fixed door. Side lites often have their own separate frame or are contained within the frame of a composite assembly.

Single glazing — glazing that is just one layer of glass or other glazing material.

Single-hung window — a hung window with only one operable sash.

Single mode — the primary window/door is closed and latched, the secondary window/door or outer primary window/door is opened fully, and the insect screen (when offered or specified by the manufacturer) is in the functional position.

Slider — see Horizontal sliding window.

Sliding door — a door that consists of manually operated door panels, one or more of which slide or roll horizontally within a common frame, and can also contain fixed lites/panels.
Note: Typically, operating panels are identified with an (X) and fixed lites or fixed panels are identified with an (O).

Sloped glazing (other than unit skylights) — a glass and framing assembly that is sloped more than 15° from the vertical and which forms essentially the entire roof of the structure.
Note: Generally, this is a single slope construction.

Sound transmission class (STC) — a single-number rating calculated in accordance with ASTM E413, using sound transmission loss values, that provides an estimate of the sound insulation performance of an interior partition in certain common sound insulation problems.
Note: The frequency range used is typical of indoor office noises.

Spacer — the linear material that separates and maintains the space between the glass surfaces of insulating glass units.

Span — the clear distance measured parallel to the length of a mullion or divider between support points.

Spandrel — the opaque areas of a building envelope which typically occur at locations of floor slabs, columns, and immediately below roof areas.

Specification — a written document often accompanying architectural drawings, giving such details as scope of work, materials to be used, installation method, required performance, and quality of work for work under contract.

Stile — a vertical member of a sash, leaf, or panel.

Storefront — a non-residential, non-load-bearing assembly of commercial entrance systems and windows usually spanning between the floor and the structure above, designed for high use/abuse and strength.
Note: Storefront systems are typically designed to accommodate field fabrication and glazing and employ exterior glazing stops at one side only. Storefront employs shallow rectilinear framing profiles (approximately 150 mm [6 in] or less), which are often made available in “stock lengths”. Vertical framing members run between the top of the floor slab and structure above, with provision for anchorage at all perimeter conditions. Operating vents and entrance doors are provided as separate inserts.

Storm door — see Secondary door.

Storm window — see Secondary window.

Structural test pressure (STP) — the pressure differential applied to a window, door system, TDD, roof window, SSP, or unit skylight.
Note: Structural test pressure (STP) is not to be confused with design pressure (DP) or Performance Grade (PG).

Sunroom — a multi-sided structure consisting of a high percentage of glazed area versus framing area.
Note: Usually a non-conditioned area attached to the exterior of an existing building.

System — the parts, components, hardware, and/or accessories that yield a complete, fully functional assembly.

Tempered glass — see Fully tempered glass.

Test specimen — a complete, fully functioning window, door, SSP, TDD, roof window, or unit skylight supplied by the applicant and fitted in the test apparatus in accordance with the manufacturer’s written installation instructions (including the manufacturer’s instructions for clearance, shimming, and anchoring).

Thermal barrier — an element made of material with relatively low thermal conductivity, which is inserted between two members having high thermal conductivity, in order to reduce the heat transfer.

Thermal break — see Thermal barrier.

Thermoplastic — a polymer material that turns to liquid when heated and becomes solid when cooled and is able to repeat these processes.

Top-hinged window — a window consisting of sash hinged at the head which swings inward or outward using a continuous top hinge or individual hinges, primarily for cleaning or emergency escape and rescue purposes and not for ventilation.

Torsion — the twist induced in a product by the application of a static load to an extreme free corner of that product and normal to its plane.

Transom — an operable or non-operable product that is designed to be a companion product installed above a fenestration product.
Note: Transoms often have their own separate frame or are contained within the frame of a composite unit.

Tributary width — the width of wind-bearing area contributing to the load on a mullion or divider.

Tropical awning window — a window consisting of one or more top-hinged or pivoted sash that swing outward at the bottom edge and are operated by one control device that securely closes them at both jambs without the use of any additional manually controlled locking devices.

Tropical window — see Jal-awning window, Jalousie window, and Tropical awning window.

True divided lite (TDL) — a lite in which dividers (muntins) separate the glazing into individual smaller glazing lites.

Tubular daylighting device (TDD) — a non-operable fenestration unit primarily designed to transmit daylight from a roof surface to an interior space via a closed-end tubular conduit. The basic unit generally consists of an exterior glazed weathering surface, a light-transmitting tube with a reflective inner surface, and an interior closure glazing in a retainer frame. The interior closure glazing is generally sealed. A TDD product line can be tested and rated in either or both of the following configurations: (a) Closed ceiling (CC): the tubular conduit passes through unconditioned space. (b) Open ceiling (OC): the tubular conduit is suspended in conditioned space.

Turn-tilt window unit — see Dual-action window.

Unit skylight — a complete factory-assembled glass- or plastic-glazed fenestration unit consisting of not more than one panel of glass or plastic installed in a sloped or horizontal orientation primarily for natural daylighting. Unit skylights are either fixed (non-operable) or venting (operable).

Vehicular-access door — a door that is used for vehicular traffic at entrances of buildings such as garages, loading docks, parking lots, factories, and industrial plants, and is not generally used for pedestrian traffic.

Vertical fenestration — fenestration products that are installed at an angle less than 15° from vertical.

Vertically pivoted window — see Pivoted window.

Vertical sliding window — a hung or non-hung window consisting of at least one manually operated sash that slides vertically within a common frame.

Water penetration — penetration of water beyond the plane intersecting the innermost projection of the test specimen, not including interior trim and hardware, under the specified conditions of air pressure difference across the specimen.

Weatherstrip (weatherseal) — a flexible component used to reduce air leakage, water penetration, or both between sash, leaf, panel, and/or frame.

Weephole (weep) — an opening that allows water to drain.

Welded — when materials are fused by heat to become one when cooled.

Window — an operable or non-operable assembly that is installed in an opening within an exterior wall or roof intended to admit light or air to an enclosure, and is usually framed and glazed.
Note: Windows are typically designed to accommodate factory fabrication and glazing.

Window wall — a non-load-bearing fenestration system provided in combination assemblies and composite units, including transparent vision panels and/or opaque glass or metal panels, which span from the top of a floor slab to the underside of the next higher floor slab.
Note: Window walls are available with separate or integral slab edge covers and can be fabricated from windows or curtain wall or storefront systems. Primary provision for anchorage occurs at head and sill conditions. Receptor systems can be designed as a part of drainage and movement accommodation provisions.

The Language of Windows

Below are some additional terms which have been used to describe windows or doors or their component parts. These definitions are not included in current industry standards.

Anchor strip — board around a window frame nailed to house framing. It also serves as windbreak. In newer windows, anchor strip may be plastic or metal.

Angle brace — wood member nailed across window frame at upper corners while frame is in a squared position in order to maintain squareness before installation.

Apron — horizontal trim board under a window stool.

Backband (also Backbend) — millwork around outside edge of the window casing, usually installed when the casing consists of flat boards.

Barn sash — plain sash for farm or cottage, used as a fixed, sliding, or casement window; generally installed in a rough frame for utility or temporary structures.

Bay window — windows that project out from the wall and extend to the ground. An “angle bay window” refers to the angle of departure from the plane of the wall.

Bead (also bead stop; stop) — wood strip against which a swinging sash closes, as in a casement window. Also, a finishing trim at the sides and top of the frame to hold the sash, e.g., a fixed sash or a double-hung window sash.

Bedding — method of glazing in which a thin layer of putty or glazing compound is placed in the glass rabbet, the glass pressed into the bed, the glazier’s points (metal tabs) driven, and the sash is face-puttied over the points.

Blank window — see false window.

Bottom rail — bottom horizontal member of a window sash.

Bow window (also compass, radial bay window) — rounded bay window that projects from a wall in the shape of an arc. It consists usually of five sash.

Boxed mullion — hollow mullion between two double-hung windows to hold sash weights.

Box-head window — window made so the sash can slide vertically into the wall space above the header.

Cabinet window — projecting window for the display of goods, as in a retail store.

Cameo window — fixed oval window, generally with surrounding moldings and ornaments, often found on Colonial Revival Houses.

Cames — lead strips which hold small pieces of glass in leaded windows.

Cap — decorative molded projection, or cornice, covering the lintel of a window.

Casement adjuster — device to hold a casement window in any open position.

Casement stay — bar for holding a casement window in any of several fixed open positions.

Casing — a trim. Exposed molding or framing around a window or door, on either the inside or outside, to cover the space between the window frame or jamb and the wall.

Center-hung sash — a sash that pivots on pins in the middle of the sash stiles and sides of the window frame to allow access for cleaning from the inside.

Chicago window — a large fixed sash flanked by a narrow, often movable, sash on either side. First used by Chicago School architects in the late 19th and early 20th Century.

Clerestory (also clearstory; high-light window) — a window in the upper part of a lofty room that admits light to the center of a room.

Colonial windows — windows with small rectangular panes, or divided lites, designated as 12-lite, 16-lite and so on.

Corner window — two windows meeting at a corner of a structure.

Coupled window — two separate windows separated by a mullion. Also called a double window.

Diffusing glass — glass with an irregular surface for scattering light; used for privacy or to reduce glare.

Diocletian window — semi-circular window divided by wide mullions into three lights (lites). This ancient Roman style was later used by Palladio in the 16th century. Also called a Therm. Used in Classical Revival buildings of the early 1900s.

Dormer window — window in a wall that either projects from a sloping roof, or is recessed (inset dormer) into the roof, or a combination of both.

Drip cap — horizontal molding to divert water from the top casing so water drips beyond the outside of the frame.

Drop window — vertical window in which the sash can descend into a cavity in the wall below the sill.

Extension blind stop — molded window frame member, usually the same thickness as the blind stop and united with it, thus increasing the width of the blind stop, in order to close the gap between the window frame and the rough opening in the house frame. Used to attach the window frame to the wood framing. Also known as blind stop extender or blind casing.

Extension jamb (also jamb lining and jamb extender) — a board used to increase the depth of the jambs of a window frame to fit a wall of any given thickness.

Eyebrow windows — low, inward-opening windows with a bottom-hinged sash. Usually attic windows built into the top molding of the house, the units sometimes are called “lie-on-your-stomach” windows or slave windows. Often found in Greek Revival and Italianate houses.

Face glazing — common glazing set with putty in a rabbeted frame.

Fanlight (also sunburst light; fan window; circle-top transom) — a half-circle window over a door or window, with radiating bars.

Fire window — window with fire-endurance rating specified for the location.

Foil — lobe on a leaf-shaped curve formed by the cusping of a circle or arch. The number of foils involved is indicated by a prefix, e.g., tre-foil (3); quatre-foil (4), etc. Foils are found in windows of Gothic Revival churches and houses.

Folding casement — casement windows hinged together so they may fold into a confined space.

French window — two casement sash hinged on the sides to open in the middle; sash extends to the floor and serves as a door to a porch or terrace.

Georgian window — a double-hung window.

Glazing bead (also glass stop and wood stop and sill bead) — removable trim that holds glass in place.

Glazing channel — groove cut into sash for acceptance of glass.

Glazing clip — metal clip for holding glass in a metal frame while putty is applied.

Glazing gasket — special extruded plastic shape for attaching window glass to metal or masonry wall openings. It also serves as a cushion and insulator.

Gothic-head window — window topped with a pointed arch. It is not as tall and narrow as the pure Gothic Lancet window.

Guillotine window — the first double-sash window, with only one movable sash and no counterweights or balancing system. A peg was inserted through a hole in the movable sash and into a corresponding hole in the frame. Its tendency to come slamming down led to the colorful name.

Hanging sash (also hung sash) — sash hung on a cord connected to a counterweight.

Header (also lintel; beam) — supporting member or beam above window opening which transfers building weight above to the supporting wall structure on each side of the window. The term header is generally in reference to a wood beam, whereas “Lintel” often refers to a steel beam.

Head flashing — flashing installed in a wall over a window.

Hit-and-miss window — two-part window with the lower sash containing movable ventilation panels.

Inset dormer (recessed, internal) — see dormer window.

Interior glazed — glazing installed from inside of the building structure.

Jamb depth — width of the window frame from inside to outside.

Label — a projecting molding by the sides and over the top of an opening.

Label stop — ornamental projection on each end of a label, sill, or sill course. Often takes the shape of a gargoyle or other decorative carving.

Lancet window — tall, narrow window with a pointed-arch top, often with leaded diamond shaped lights; characteristic of Gothic architecture.

Lattice window (also lozenge) — window with glazing bars set diagonally.

Lead light (also lead glazing; stained glass) — window with small panes of glass set in grooved rods of cast lead or came. The glass may be clear, colored, or stained.

Lintel — horizontal member (wood, steel, or stone) over a window opening to support the weight of the wall above. A header.

Loop window (also Balistraria) — a long and narrow vertical opening, usually widening inward, cut in a medieval wall, parapet, or fortification for use by archers. Modifications appear in Romanesque Revival architecture.

Mold stone (also jamb stone) — a stone that serves as a window jamb.

North-light roof — sawtooth roof with north-facing clerestory windows.

Ogee curve (also ogee molding) — reverse flex curve commonly found in window moldings and trim pieces.

Operable transom — panel usually glazed over a door which may be opened for ventilation.

Operable window — window that can be opened for ventilation.

Oriel window — a window projecting from the wall and carried on brackets, corbels, or a cantilever. Unlike a bay window, the projection of an oriel does not extend all the way to the ground.

Panel window — a form of picture window consisting of several sash or fixed glazings, separated by crossbars or mullions–or both.

Palladian window — tripartite window by the 16th century Italian architect Andrea Palladio.

Parting bead (also parting strip and parting stop) — a vertical strip on each jamb that separates the sashes of a double-hung window.

Parting slip — a thin wood strip separating the sash weights in the weight box of each jamb of old double-hung windows.

Picture window — large fixed windows.

Prime sash — balanced or moving sash of a window unit.

Projected window — awning type window that swings either inwards or outwards at the top or the bottom.

Quarrel — a diamond- or square-shaped glass piece set diagonally. A medieval term for small panes of glass set diagonally in Gothic windows.

Queen Anne window — a window with small glass windows or lights arranged in various forms, usually only on the upper sash. Appeared 1870s.

Reglet — plastic or wood molding put in a concrete or masonry opening for a uniform groove for a spline-type gasket to hold window glass.

Reversible extension blind stop — an extension blind stop that is rabbetted to receive l/2 or 25/32-in. sheathing.

Saddle bar — light steel bar placed horizontally across a window to stiffen leaded glazing.

Saddle bead — glazing bead for securing two panes.

Shading coefficient — decimal value which is the solar gain of a window, divided by the solar gain for a clear single-glass window of the same size. The shading coefficient of clear, double-glazing is about 0.85 to 0.9.

Sill (also sill plate; inside sill; outside sill) — horizontal member at the bottom of the window frame; a masonry sill or sub-sill can be below the sill of the window unit.

Sill drip molding — sill member on a window frame serving as a screen stop; also the extension of the sill that contains the drip cut.

Solid frame — window frame made from a single piece of lumber.

Splayed window — window unit set at an angle in a wall.

Storm clip — device attached to the muntin of a metal sash to stop the pane from moving outwards.

Transom light — window sash located above a door.

Triple window — generally refers to any tripartite group of windows with square heads. Found on Colonial Revival houses. Units suggest Palladian windows but are less expensive to construct.

Venetian window — same as Palladian window.

View sash — picture window with the lights divided by muntins.

Wash cut — beveled cut in a stone sill to divert water.

Water drip — molding sometimes used on exterior surfaces of an in-swinging casement sash to prevent water from being driven over the sill.

Weep cut (also drip cut) — groove in the underside of a horizontal board or masonry unit which projects beyond the wall service below to prevent water from moving back toward the wall surface

Yoke — head window jamb in a box window frame.

Yorkshire light — window with one or more fixed sash and a horizontally moving sash.