Someone who intentionally attacks his own players in an online game.
Teamkilling in games originated by people exploiting game servers that accepted 'friendly fire'.
More recently, celebrity gaming figures like LOLMAN
have utilised impact circles of weapons so as to become profficient at teamkilling, even without 'friendly fire' enabled. In games like Unreal Tournament
, this works by firing a projectile in the right place, so that the force of impact makes a teammate fly so far that he loses a frag due to fall damage. This is a 'teamkill'.