What is a Fixture?

A fixture refers to any type of theatrical light. This might mean a conventional light, such as a SourceFour 19 Degree. It could mean an intelligent LED. It could mean a moving head. It could be used to describe lights that haven’t even been invented yet. They’re all fixtures.

Now, people might use other terms.
Light.
Unit.
Luminaire.

Same thing. There are certainly nuances to each of these terms, but a light by any other name still shines as brightly.