Knockout (K.O.), or simply knock, is a winning criterion of many ringsports , such as boxing, kickboxing, Muay Thai and others. A knockout victory is declared when one participant is unable to rise from the canvas within a specified period of time. A referee can also declare a technical knockout (T.K.O.) in cases where a participant is sufficiently injured, unbalanced, or confused to be unable to continue the fight.

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.

The verb kayoed can also be used to refer to someone or something that has been knocked unconscious.

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Knockout also refers to:

• the knockout rule, also called mercy rule or slaughter rule used in various sports
• Gene knockout
• Two different British comics were called Knockout
• A knockout tournament, also called a single-elimination tournament
• Knockout whist
• Knockout Kings is a video game series