PVS
This acronym is somewhat disputed. It may stand for Potential Visibility Set, Potentially Visible Set, Potentially Viewable Set or many similar variations.
Every entity has a PVS. For players, it is the first line of defense over what geometry, entities and effects the game engine should render at any given point. Only objects within the current PVS - i.e. which are "potentially visible" - are considered for rendering, and it is cheap for the processor to decide whether or not this condition is being met.
For NPCs, a PVS is likely to be used to filter what other entities should be checked for line-of-sight. Additionally, entities will sometimes perform actions when a player enters their PVS.
A PVS consists of the visleaf that surrounds the entity and any other visleaves that are visible from it. See Optimization (Geometry)#Visibility for more information on how visleaves are created.