AAMA . American Architectural Manufacturers Association. A national trade association that establishes voluntary standards for the window, door, and skylight industry.
 

Absorptance . The ratio of radiant energy absorbed to total incident radiant energy in a glazing system.
 

Annealed glass . Standard sheet of plate glass. Annealed glass means glass that has been subjected to a slow, controlled cooling process during manufacture to control residual stresses so that it can be cut or subjected to other fabrication. Regular polished plate, float, sheet, rolled, and some patterned surface glasses are examples of annealed glass.
 

Installation Masters. A former division of AAMA, now a separate company, that trains and certifies window installers to manufacturers specifications. A highly prized certification that means a window contractor really knows what they're doing when working on your house.
 

ANSI . American National Standards Institute. Clearing house for all types of standards and specifications.

Argon. An inert, nontoxic gas used in insulating windows to reduce heat transfer. Heavier than air.
 

Awning . Window similar to a casement except the sash is hinged at the top and always swings out.
 

Balance . A mechanical device (normally spring loaded) used in single- and double-hung windows as a means of counterbalancing the weight of the sash during opening and closing.

Bay window . An arrangement of three or more individual window units, attached so as to project from the building at various angles. In a three-unit bay, the center section is normally a fixed picture window, with the end panels operable as single-hung or casement windows.
 

Bottom rail . The bottom horizontal member of a window sash.

Bow window. A rounded bay window that projects from the wall in an arc shape, commonly consisting of five sashes.

Breathing Tube. A tiny tube inserted between double or triple pane window panes when transporting newly manufactured windows up into a high altitude (3000 feet or higher). They help move trapped, expanding air caused by the altitude lift that otherwise might pop or weaken the window seals. The tube is removed upon installation. The tube also vents a very small amount of the argon gas that might have been manufacturered in, but so small an amount (perhaps 1% or less), that energy efficiency is left intact.

Brick molding. A standard milled wood trim piece that covers the gap between the window frame and masonry.

Btu (B.T.U.). An abbreviation for British Thermal Unit--the heat required to increase the temperature of one pound of water one degree Fahrenheit.

Casement . A window sash that swings open on side hinges; in-swinging are French in origin; out-swinging are from England.
 

Check rail . The bottom horizontal part of the upper sash and the top horizontal part of the lower sash which meet at the middle of a double-hung window. The "meet bar" (meet rail) where the window locks are afixed.
 

Condensation . The deposit of water vapor from the air on any cold surface whose temperature is below the dew point, such as a cold window glass or frame that is exposed to humid indoor air.
 

Conduction . Heat transfer through a solid material by contact of one molecule to the next. Heat flows from a higher-temperature area to a lower-temperature one.
 

Desiccant . An extremely porous crystalline substance used to absorb moisture from within the sealed air space of an insulating glass unit.
 

Divided lite . A window with a number of smaller panes of glass separated and held in place by muntins.

Double glazing . In general, two thicknesses of glass separated by an air space within an opening to improve insulation against heat transfer and/or sound transmission. In factory-made double glazing units, the air between the glass sheets is thoroughly dried and the space is sealed airtight, eliminating possible condensation and providing superior insulating properties. Double pane window.

Double-hung window . A window consisting of two sashes of glass operating in a rectangular frame, in which both the upper and lower halves can be slid up and down. A counterbalance mechanism usually holds the sash in place.

Double-strength glass . Sheet glass between 1/8 and 1/4 inch thick.

Energy Star. A national window, siding and appliance rating and requirement program. For windows the requirements measured are for thermal performance, structural integrity, air infiltration, water resistance and wind loads.
 

Extrusion . The process of producing vinyl or aluminum shapes by forcing heated material through an orifice in a die. Also, any item made by this process.

Eyebrow windows . Low, inward-opening windows with a bottom-hinged sash. These attic windows built into the top molding of the house are sometimes called "lie-on-your-stomach" or "slave" windows. Often found on Greek Revival and Italianate houses.

Fan lite . A half-circle window over a door or window, with radiating grids/bars. Also called circle top transom or arch window.

Fixed lite . A pane of glass installed directly into non-operating frame; also, the opening or space for a pane of glass in a non-operating frame.

Fixed panel . An inoperable panel of a sliding glass door or slider window.

Fixed window. A window with no operating sashes. Also called a picture window.

Flashing. Sheet metal or other material applied to seal and protect the joints formed by different materials or surfaces.
 

Fogging. A deposit of contamination left on the inside surface of a sealed insulating glass unit due to extremes of temperatures or failed seals. Condensation in between the panes that can't be cleaned.

Frame. The fixed frame of a window which holds the sash or casement as well as hardware.

Gas fill. A gas other than air, usually argon or krypton, placed between window or skylight glazing panes to reduce the U-factor by suppressing conduction and convection. Helps with insulating the window.

Glaze or Glazing:  NOT a film like on a car wind-shield, but instead Glazing material means glass, including annealed glass, organic coated glass, tempered glass, laminated glass, wired glass; or combinations thereof where these are used.

Glazing bead. A molding (often vinyl) or stop around the inside of a window frame to hold the glass in place.
 

Greenhouse or Garden window. A three-dimensional window that projects from the exterior wall and usually has glazing on all sides except the bottom, which serves as a shelf. Also called a garden window.

Header. The upper horizontal part of a window frame. Also called head.
 

Heat gain. The transfer of heat from outside to inside by means of conduction, convection, and radiation through all surfaces of a house.

Heat loss. The transfer of heat from inside to outside by means of conduction, convection, and radiation through all surfaces of a house.
 

Hinged windows. Windows (casement, awning, and hopper) with an operating sash that has hinges on one side. See also Projected window.

Hopper. Window with sash hinged at the bottom.

Horizontal slider. A window with a movable panel that slides horizontally. Can be a single slider (one panel moves) or a double slider (both panels can move).

Infiltration. The movement of outdoor air into the interior of a building through cracks around windows and doors or in walls, roofs, and floors.

Infrared radiation. Invisible, electromagnetic radiation beyond red light on the spectrum, with wavelengths greater than 0.7 microns.

Insulated shutters. Insulating panels that cover a window opening to reduce heat loss.

Insulating glass. Two or more pieces of glass spaced apart and hermetically sealed to form a single glazed unit with one or more air spaces in between. Also called double glazing or double paned windows.
 

Jalousie. Window made up of horizontally-mounted louvered glass slats that abut each other tightly when closed and rotate outward when cranked open. Often found in older California stucco type homes.

Jamb. A vertical member at the side of a window frame, or the horizontal member at the top of the window frame, as in head jamb.

Laminated glass. Two or more sheets of glass with an inner layer of transparent plastic to which the glass adheres if broken. Used for safety glazing and sound reduction. Often used in hurrincane areas.

Lift. Handle for raising the lower sash in a double-hung window. Also called sash lift.

Lite. A window; a pane of glass within a window. Double-hung windows are designated by the number of lights in upper and lower sash, as in six-over-six.

Lintel. A horizontal header beam above a window or door opening that supports the structure above and that span a door opening; may be structural or solely decorative
 

Long-wave infrared radiation. Invisible radiation, IR,  beyond red light on the electromagnetic spectrum (above 3.5 micro meters), emitted by warm surfaces such as a body at room temperature radiating to a cold window surface. In thermography photos, can show where heat is coming in and out of house structures and windows.
 

Low-emittance/emissivity (low-E) coating. Microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. A typical type of low-E coating will let in normal light through a window but refract some heat back outside. 

Meeting rail. The part of a sliding glass door, a sliding window, or a hung window where two panels meet and create a weather barrier. The "meet bar" also known as check rail.

Metal or Vinyl clad windows. Exterior wood parts covered with extruded aluminum, other metal, or vinyl with a factory-applied finish to deter the elements.
 

Mullion. A major structural vertical or horizontal member between window units or sliding glass doors. Used when two or more separate windows are installed into a single opening. This bar supports where the windows meet in the opening.
 

Muntin. A secondary framing member (horizontal, vertical, or diagonal) to hold the window panes in the sash. This term is often confused with mullion. Muntins are used in mult-lite windows and can often be seen in the old Victorian style home windows.
 

Muntin grilles. Wood, plastic, or metal grids designed for a single-lite sash to give the appearance of muntins in a multilight sash, but sometimes removable for ease in cleaning the window. They are also called grids-- colonial grids is one style that can be installed between the two panes in a double paned vinyl window.
 

Nailing fin. An integral extension of a window or patio door frame which generally laps over the conventional stud construction and through which nails are driven to secure the frame in place. These are used when putting windows into a new building, but are cut off the new windows and not used when replacing old windows with new ones.

NFRC. National Fenestration Rating Council. Like the AAMA and Energy Star programs, provides standards and guidelines for windows for thermal performance, structural integrity, air infiltration, water resistance and wind loads.

Obscure glass. Any textured glass (frosted, etched, fluted, ground, etc.) used for privacy, light diffusion, or decorative effects. Often used in bathroom windows.
 

Operable window. Window that can be opened for ventilation.
 

Operator. Crank-operated device for opening and closing casement or jalousie windows.
 

Pane. One of the compartments of a door or window consisting of a single sheet of glass in a frame; also, a sheet of glass.
 

Panel. A major component of a sliding glass door, consisting of a lite of glass in a frame installed within the main (or outer) frame of the door. A panel may be sliding or fixed.
 

Picture window. A large, fixed window framed so that it is usually, but not always, longer horizontally than vertically to provide a panoramic view. Cannot open.
 

Polyvinylchloride (PVC). An extruded or molded plastic material used for window framing and as a thermal barrier for aluminum windows. Called vinyl.
 

R-value. A measure of the resistance of a glazing material or fenestration assembly to heat flow. It is the inverse of the U-factor (R = 1/U) and is expressed in units of hr-sq ft-°F/Btu. A high-R-value window has a greater resistance to heat flow and a higher insulating value than one with a low R-value.
 

Radiation. The transfer of heat in the form of electromagnetic waves from one separate surface to another. Energy from the sun reaches the earth by radiation, and a person's body can lose heat to a cold window or skylight surface in a similar way.
 

Rail. Horizontal part of a window sash.

Reflectance. The ratio of reflected radiant energy to incident radiant energy.
 

Reflective glass. Window glass coated to reflect radiation striking the surface of the glass. Different than mirrored glass, reflective glass allows normal sight in and out of the window.
 

Retrofitting. Adding or replacing items on existing buildings. Typical retrofit products are replacement doors and windows, insulation, storm windows, caulking, weatherstripping, vents, landscaping.
 

Rough opening. The opening in a wall into which a door or window is to be installed.
 

Safety glass. A strengthened or reinforced glass that is less subject to breakage or splintering. Also known as tempered glass.
 

Sash. The portion of a window that includes the glass and the framing sections directly attached to the glass, not to be confused with the complete frame into which the sash sections are fitted and that touch the house studs and walls.
 

Screen. Woven mesh of metal, plastic, or fiberglass stretched over a window opening to permit air to pass through, but not insects.
 

Shading coefficient (SC). A measure of the ability of a window or skylight to transmit solar heat, relative to that ability for 1/8-inch clear, double- strength, single glass. It is being phased out in favor of the solar heat gain coefficient, and is approximately equal to the SHGC multiplied by 1.15. It is expressed as a number without units between 0 and 1. The lower a window's solar heat gain coefficient or shading coefficient, the less solar heat it transmits, and the greater is its shading ability.
 

Short-wave infrared radiation. Invisible radiation, just beyond red light on the electromagnetic spectrum (between 0.7 and 2.5 microns), emitted by hot surfaces and included in solar radiation.
 

Sill. The lowest horizontal part in a door, window, or sash frame. It usually sticks out like a decorative shelf.

Sill track. The track provided at the sill of a sliding glass door. Also, the sill part incorporating such a track.
 

Simulated divided lites. A window that has the appearance of a number of smaller panes of glass separated by muntins, but actually is a larger glazing unit with the muntins placed between or on the surfaces of the glass layers.
 

Single glazing. Single thickness of glass in a window or door. A single pane window.
 

Single-hung window. A window consisting of two sashes of glass, the top one stationary and the bottom movable.

Single-strength glass. Glass with thickness between 1/12 and 1/10 inch thickness-- rarely found in newer windows for sale but often found in 40 year or older windows and storm windows.

Sliding glass door. A door fitted with one or more panels that move horizontally on a track and/or in grooves. Moving action is usually of rolling type (rather than sliding type). Also called gliding door, rolling glass door, and patio sliding door.
 

Sliding window. A window fitted with one or more sashes opening by sliding horizontally in grooves.  Sliders can be single sliders (one panel moves) or double sliders (both panels move).

Solar control coatings. Thin film coatings on glass or plastic that absorb or reflect solar energy, thereby reducing solar gain.
 

Solar heat gain coefficient (SHGC). The fraction of solar radiation admitted through a window or skylight, both directly transmitted, and absorbed and subsequently released inward. The solar heat gain coefficient has replaced the shading coefficient as the standard indicator of a window's shading ability. It is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits, and the greater its shading ability. SHGC can be expressed in terms of the glass alone or can refer to the entire window assembly.
 

Solar radiation. The total radiant energy from the sun, including ultraviolet (UV) and infrared wave (IR) lengths as well as visible light.

Solar spectrum. The intensity variation of sunlight across its spectral range.
 

Spectrally selective glazing. A coated or tinted glazing (ie: pane of glass) with optical properties that are transparent to some wavelengths of energy and reflective to others. Typical spectrally selective coatings are transparent to visible light and reflect short-wave and long-wave infrared radiation.
 

Tempered glass. Treated glass that is strengthened by reheating it to just below the melting point and then suddenly cooling it. When shattered, it breaks into small pieces. Approximately five times stronger than standard annealed glass; is required as safety glazing in patio doors, entrance doors, side lights, and other hazardous locations. It cannot be recut after tempering. Also called safety glass.
 

Thermal break. An element of low conductance placed between elements of higher conductance to reduce the flow of heat. Often used in aluminum windows but now found in virtually every type double paned window.

Threshold. The part of the frame that lies at the bottom of a sliding glass door or swinging door; the sill of a doorway.
 

Tilt window. A single- or double-hung window whose operable sash can be tilted into the room for interior washability.
 

Transmittance. The percentage of radiation that can pass through glass coatings. Transmittance can be defined for different types of light or energy, e.g., visible light transmittance, UV transmittance, or total solar energy transmittance.

Transom window. The window located above a door. Also called transom lite.
 

Triple glazing. Three panes of glass or plastic with two air spaces between. Triple paned window.
 

U-factor (U-value). A measure of the rate of non-solar heat loss or gain through a material or assembly. It is expressed in units of Btu/hr-sq ft-°F (W/sq m-°C). Values are normally given for NFRC/ASHRAE winter conditions of 0° F (18° C) outdoor temperature, 70° F (21° C) indoor temperature, 15 mph wind, and no solar load. The U-factor may be expressed for the glass alone or the entire window, which includes the effect of the frame and the spacer materials. The lower the U-factor, the greater a window's resistance to heat flow and the better its insulating value.
 

Ultraviolet light (UV). The invisible rays of the spectrum that are outside of the visible spectrum at its short-wavelength violet end. Ultraviolet rays are found in everyday sunlight and can cause fading of paint finishes, carpets, and fabrics.
 

Vent. The movable framework or sash in a window that is hinged or pivoted to swing open. Usually part of a casement window.

Vinyl-clad window. A window with exterior wood parts covered with extruded vinyl.
 

Visible light. The portion of the electromagnetic spectrum that produces light that can be seen. Wavelengths range from 380 to 720 nanometers. The normal light that comes through windows.
 

Visible transmittance (VT). The percentage or fraction of the visible spectrum (380 to 720 nanometers) weighted by the sensitivity of the eye, that is transmitted through the panes of glass of a window. 
 

Warranty. Usually written guarantee of the integrity of a product and of the maker's responsibility for the repair or replacement of defective parts. Coverage might or might not include both parts and labor to fix a broken window.
 

Warm-edge technology. The use of low-conductance spacers or adhesives around spacers to reduce heat transfer near the edge of insulated window panes.
 

Weatherstripping. A strip of resilient material for covering the joint between the window sash and frame in order to reduce air leaks and prevent water from entering the house.
 

Weep hole. A small opening in the outer part of a window frame through which water may drain to the building exterior.