In online multiplayer games, when somebody else kills the enemy you are fighting. This often results in you earning less experience points than you normally would for killing that enemy because two people were attacking it.
Only noobs do this, and is considered a gamer's equivalent of a war crime among many.
"Dude, I was fighting a (insert enemy) and this noob came out of nowhere and stole my kill."
_ _ _
"Don't kill steal, asshole!"
When you playing an online game that involves killing, and right when your about to kill someone, someone jumps out and kills him first
player 1: OHHH IMA KILL YOU
player 2: NOOOO
player 3: hahahah KILL STEAL!!
player 1: WTF U HAXZOR ROFLCOPTER ROFLCAKES!!!