Portal culling
Visibility technique that carves a scene into rooms linked by portals and draws only what the camera can see through them.
The scene is partitioned into cells — usually rooms — and portals are the openings between them: doors, windows, arches. Rendering starts in the camera's cell and walks recursively outward, but only through portals that fall inside the running, clipped view frustum. If a portal lies outside it, the cell behind it and everything beyond is skipped. Indoors, where walls hide most of the world anyway, a handful of plane tests cull whole rooms at a stroke.
Seth Teller formalised the idea in his 1992 thesis, and it carried the indoor engines of the 1990s. Precompute the visibility instead of testing per frame and you get a PVS; occlusion culling solves the same problem more generally but at higher cost.