Occluder
A piece of geometry that hides others and is therefore used to avoid drawing what cannot be seen behind it.
Frustum culling throws away everything outside the camera, but not walls that hide the rooms behind them. Occlusion culling does, and the key is the occluder: a dense, blocking object — a building wall, a mountain — that the engine renders or rasterizes first to test what falls entirely in its shadow. Anything behind can be skipped.
Occluders are chosen with care; only large, solid surfaces pay off, and many engines let artists mark them by hand or build simplified "occluder geometry" alongside the visible mesh. Techniques range from software rasterization on the CPU through hardware occlusion queries to the hierarchical z-buffer. The idea is a cousin of antiportals — surfaces defined by what they hide rather than what they let through.