PVS

Potentially Viewable Set - a technique for culling used on server and client to exclude entities that are not seen from a position of a player.

PVS is using precomputated visibility data that is calculated at the VIS stage of map compilation.

Console variables

 r_lockpvs : disables PVS switching so you can walk around and inspect what is visible from locked position (anything not in PVS will not be drawn).