In Halo 3, the unofficial term of assassination by shooting someone in the back of the head (and sometime the back) from behind with a Rocket Launcher (or R.L. substitute), often occurs during Team RL.
An alternate term, is when you try to assassinate anyone with a weapon that clearly isn't designed for that (i.e. assassinate someone with a grenade launcher), and successfully do so.
Red Spartan: W00t! (jumping up and down while shooting)
Blue Elite: (from behind) FORE!!! (shoots from behind)
Red Spartan: X_X
Blue Elite: Boom! Explozation! :D
Random Jackass: I am evil! >:D
Random Guy: (hits him from a typical sniper-style long distance, with a RPG)
Random Jackass: Nooooo! T.T
Random Guy: Explozation, bitch!