What is housing mean for a lighting fixture of stage?
It’s the main body of a stage lighting instrument which holds all the other components of a fixture like lens, yoke, gel frame holder, c-clamp, etc. Housing materials can be plastic, steel, metal, and aluminum, the newest and best technology for making lamp housing is Acurad die casting.
The purpose of housing is for protecting inside electronic components and opening a hole for lens to focus lights. Today’s housing should be more light in order to install to stage truss and use easily for stage designers. The appearance and shape of lamp housing can be designed particularly for human’s eyes, for example pumpkin shape.
See the more professional explanation on Wikipedia.