Fixtures / Light Fixtures / Luminaires
A fixture or luminaire are both interchangeable terms used to describe lighting units. These terms often reference just the structure used to position and protect the light. Technically, a light fixture or luminaire includes the light, all the parts to position and protect the light, and the wiring to the power supply coming into the light fixture. The NEC uses the term luminaire instead of the older term fixture, but you will likely see both of these terms used at some point if you're doing electrical work.