A knockout (also referred to as a K.O.) is a fight-ending, winning criterion in several full-contact combat sports, such as boxing, kickboxing, Muay Thai, mixed martial arts, Karate and other sports involving striking. A full knockout is considered any legal strike or combination thereof that renders an opponent unable to continue fighting.
The term is often associated with a sudden traumatic loss of consciousness caused by a physical blow. Single powerful blows to the head (particularly the jawline and temple) can produce a cerebral concussion and cause a sudden, dramatic KO. Body blows, particularly the liver punch, can cause progressive, debilitating pain that can also result in a KO.
In boxing, kickboxing, etc. a knockout is usually awarded when one participant falls to the canvas and is unable to rise to his feet within a specified period of time, typically because of exhaustion, pain, disorientation, or unconsciousness. For example, if a boxer is knocked down and is unable to continue the fight within a ten second count, he is counted as having been knocked out and his opponent is awarded the KO victory.
In mixed martial arts competitions, no time count is given after a knockdown, as the sport allows submission grappling as well as ground and pound. If a fighter loses consciousness ("goes limp") as a result of legal strikes, or does not immediately defend himself after a knockdown, he is declared knocked out. As many MMA fights can take place on the mat rather than standing, it is possible to score a KO via ground and pound, a common victory for grapplers.