Fight-ending, winning criterion in certain full-contact combat sports
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.
https://en.wikipedia.org/wiki/Knockout