knockout

right|200px|thumb|Ingemar Johansson knocks [[Floyd Patterson out, becoming boxing heavyweight champion of the world, on June 26, 1959.]] right|200px|thumb|A heavy blow to the head is a frequent cause of a knockout. Muhammad Ali delivers one to [[Brian London and retains his heavyweight championship by third-round KO on August 6, 1966.]] A knockout (abbreviated to KO or K.O.) is a fight-ending, winning criterion in several full-contact combat sports, such as boxing, kickboxing, Muay Thai, mixed martial arts, karate, some forms of taekwondo and other sports involving striking, as well as fighting-based video games. 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 or a carotid sinus reflex with syncope and cause a sudden, dramatic KO. Body blows, particularly the liver punch, can cause progressive, debilitating pain that can also result in a KO.