Betrayal

In Halo terminology, a betrayal is the occurance of a player in a team-active game who kills another player who is on his or her team.

Background
In order for a betrayal to occur, the player must be completely responsible for the death of his or her teammate. In other words, in most cases the enemy team(s) can play little no role in the act. For example, if two players on opposing teams are battling each other, and a third player causes his teammate (who has already been damaged) to die because of a misfired grenade, the enemy will be credited with the kill.

Most betrayals occur with the use of explosives, due to the factor of splash damage. Almost all betrayals using non-splash damaging ammo are deliberate. In Halo 2, deliberate team-killers were a great hassle to deal with, though such an offense was not "bannable". In Halo 3, there is an option called "Betrayal Booting", in that even in matchmade games where booting a player is not normally allowed, anyone who commits enough betrayals may be booted by the last teammate who they killed. In Custom Games, "Betrayal Booting" can be set manually to decide how many betrayals must occur before the player is booted.