PVS (potentially visible set)
A precomputed table of which parts of a static world can be seen from each other part — expensive to bake, nearly free to look up.
The PVS is computed ahead of time: for each region of the world it works out which other regions could be visible from it at all. At runtime the engine only looks up which region the camera is in and draws just what that region's set contains. The rest is never touched.
Quake (1996) computed its PVS from the BSP tree when the level was compiled — a heavy bake that could take hours but reduced the runtime visibility test to a plain table lookup. The catch is that everything must be static; moving geometry has no place in the precomputed set.