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One Hit Kill is a term that refers to abilities and attacks that are explicitly meant to kill the target with a single use. This can happen any variety of ways, but the target must be struck down and killed with a single use of the corresponding ability to qualify, so this term does not apply to attacks that instantly kill the target after multiple stages.
OHK powers are generally considered to be broken since they often involve some manner of Durability Negation or a major hax component that makes them very hard to counter.
However, several of them have limitations on what they're able to kill or how they're able to kill a target (i.e. Akame's Murasame will only work on living, organic beings with a heart).