A one-hit kill move (also “fatality”, “instakill”, etc.) is a move that will guarantee a K.O. if it hits. It doesn’t matter how much life the opponent has left; if it works, it’s all over.
Some games have this as a system; usually it is very difficult to land even with setups (because if the requirements aren’t very tough, the game balance is pretty much nonexistent... ever play Time Killers or BloodStorm?). Some games with them aren’t completely awful: Aggressors of Dark Kombat, Guilty Gear, Hokuto no Ken, Samurai Shodown V Special, and Sengoku Basara X all have them.