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KO redirects here. For other uses, see Ko. For other uses of knockout, see Knockout (disambiguation).

A boxer is knocked down and receives the 10-count.

A knockout (also referred to as a K.O.) is a winning criterion in several full-contact combat sports, such as boxing, kickboxing, Muay Thai, mixed martial arts and others sports involving striking. A knockout is usually awarded when one participant is unable to rise from the canvas within a specified period of time, typically because of fatigue, injury (serious or temporarily incapacitating, e.g. a bleeding cut above the eye can blind a fighter), loss of balance, or unconsciousness. A technical knockout (also referred to as a T.K.O.) is often declared when the referee or other judges (such as official ring physician, the fighter himself, or the fighter's cornermen) decide that a fighter cannot continue the match, even though he did not fail the count.^[1] British records refer to TKOs as either "retired", if the fighter refuses to continue, or "R.S.F.", for Referee Stopped Fight.

A Cambodian Pradal Serey kickboxer is still knocked down while his adversary is being proclaimed the winner

There is actually very little known about what exactly causes a knockout but many agree it has to do with minor trauma to the brain stem. This usually happens when the head rotates sharply, often caused by a strike. A common misconception is that a knockout is caused by compression put on the carotid arteries in the neck restricting blood flow, but this is in fact how a choke out works. ^[2] A basic principle of boxing is to defend against this vulnerability by keeping both hands raised about the face.

A fighter who suffers a concussion and becomes unconscious from a strike with sufficient knockout power is referred to as having been knocked out or kayoed (KO'd). Losing balance without losing consciousness is referred to as being knocked down ("down but not out"). Repeated blows to the head are known to gradually cause permanent brain damage, with Alzheimer- or Parkinson's-like symptoms, and in severe cases may cause strokes or paralysis. This is commonly known as becoming "punch-drunk". Because of this, many physicians advise against sports involving knockouts.^[3]

A knockout can be characterized by temporary unconsciousness.

In boxing, a knockout occurs when a fighter is knocked down and does not get up within 10 seconds, as counted by the referee.

In mixed martial arts, the definitions of knockouts and technical knockouts vary according to organization and jurisdiction. According to its publicized rules, the Japanese PRIDE FC promotion does not declare 'knockout' as an official type of victory, but rather only technical knockouts when the referee considers a fighter unable to continue^[4]. Under New Jersey's Unified Rules of Mixed Martial Arts, which has become the de facto standard of MMA rules in the United States, a knockout is defined as a "failure to rise from the canvas"^[5]. In Pancrase, a knockout is declared when the competitor loses consciousness, and a technical knockout when the referee or judges consider a fighter unable to continue the match^[6].