IESNA Definition: a complete lighting unit consisting of a lamp or lamps and a ballast (when applicable) together with the parts designed to distribute the light, to position and protect the lamps, and to connect the lamps to the power supply.

A luminaire, commonly called a light fixture, contains a housing, lamp(s), socket(s), and electrical wiring (including ballasts for fluorescent and HID sources, and transformers for low voltage incandescent sources). In addition to these parts, a luminaire may also contain one or more of the following components: reflector, lens, diffuser, louver, gasket, latch, decorative trim, or mounting hardware - to provide protection; improve efficiency, appearance, or service; control glare; or affect distribution.

Adapted from The Lighting Pattern Book for Homes	Photo: Cindy Foor
Typical components of a recessed downlight.

Adapted from The Lighting Pattern Book for Homes	Photo: Cindy Foor
Typical components of a 2 ft x 4 ft recessed troffer. A troffer can have either louvers or lenses/diffusers.