Culling
Also said as Frustum culling, Occlusion culling, Potentially visible set
Not drawing what you can't see. Frustum culling skips things outside the camera's view. Occlusion culling skips things hidden behind something else, which is harder to work out.
Most of a level is behind you or behind a wall at any moment. Sending it to the graphics card wastes the frame on pictures that get painted over or never appear.
Say it
“Frustum cull everything per object, and bake occlusion for the interiors. When we enter a building, the whole street behind the wall should stop drawing.”
Where you’ve seen it
Quake works out occlusion ahead of time: its map tool stores, for each region of a level, the set of regions that could possibly be visible from it, and then it applies frustum culling to those.
Not to be confused with level of detail
Culling removes things you can't see. Level of detail simplifies things you can.