The act of causing the death of another gamer's avatar in a virtual world, often in defiance of the world's normal rules of conduct or in spite of preventative code. Often abbreviated "PK," serving as a verb, a noun, or even an adjective describing certain players or game rule sets.
The phrase most likely arose in the development of early MUDs in the 1980's where game administrators waged a continuous struggle with players bent on defying in-game authorities and their rules, regardless of the repercussions to the games' stability.
"Player killing individuals who lack the means to defend themselves is a rather dishonorable means of showcasing one's gameplaying prowess."
"Player killing is not against the charter on this server." -Anonymous source, Rallos Zek, Everquest Classic