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Knockout (K.O.), or simply knock, is a winning criterium of many ringsports, such as boxing, kickboxing, muay thai and others. A knocked participant is either unconscious or sufficiently unbalanced and confused to be unable to continue the fight for the time being, technical knockout (T.K.O.).

Since knockouts are not always possible to achieve, other winning criteria are usually employed as well.

Repeated blows to the head are known to produce damage to the cerebrum, such as concussion, and in extreme cases internal bleeding within the brain. Because of this, many physicians advise against knockout sports.

In certain other sports, there is a knockout rule, also called mercy rule or slaughter rule, which can bring a lopsided game to an early conclusion. See slaughter rule.

"Knockout" is also a common abbreviation for gene knockout.