A device used to light a stage, also known as a fixture by some. In the UK, the most common name is lantern. Probably the most correct name, and the one most used by manufacturers, is luminaire.
In a realistic setting, instruments (PARs, Fresnels, Lekos, and so on) are not meant to be seen by the audience. When the fixture is seen (for example, a chandelier, a wall sconce, or the thing on your end table) by the audience, it is called a practical.