A mob (Mobile Object) is a type of non-player character or monster in a video game which has the ability to move around. The word is considered slang.
Killing mobs can give such rewards as experience points and items. Usually the combat is player initiated; however, if the mob is flagged as aggressive, it is configured so that it will challenge a player who enters within a certain range. This area is known as the aggro range, aggro radius or an aggro line. A player engaged by such a mob is said to have aggro. This marks that player as the primary target of the attacking mob(s). Combat between players and mobs is called player versus monster (PvM) or in a broader sense, player versus environment (PvE), as opposed to player versus player (PvP) battles where the emphasis is on defeating an opposing player.