PK is an abbreviation of the English phrase "Player Killing / Player Kill". This term is often used in MMO and other multiplayer games to describe a situation where a player kills another player.  Often, games allow or even encourage PK as part of the gameplay, but in some gaming communities, it may be considered unacceptable or toxic behavior. Some games have mechanics to regulate or penalize players who engage in PK, including reputation systems, "safe zones" where PK is not allowed, or even mechanics that penalize players for excessive killing of other players.