Like a Kill, except instead of killing an enemy to get it, you kill a teammate. This is much easier, becasue they don't expect it. You can, for example, walk up to them and Pistol whip them to death. But, once they're onto you, it gets alot harder. And a name like Rampant Teamkiller doesn't exactly help.
Pvt_ryn272 was sitting on the ridge of a hill when Rampant, his teammate, walked up behind him and emptied his mp44 into Pvt_ryn's ass. Rampant was awarded with a team kill, and walked away a much better person.