(noun)
the action of striking someone with a hard blow (knocking them out) and rendering them unconscious, especially in a boxing match, or a blow that causes someone to be knocked out, or the end of a boxing match brought about when one opponent is struck sufficiently hard that they take too long to resume boxing;
e.g. He delivered the man a knockout punch. The bell sounded, signalling knockout and making her the winner of the match.
(noun)
a person or thing that is extremely attractive, appealing, or impressive in a striking or sensational way;
e.g. She's a real knockout in those heels, and looks completely amazing.
(noun)
in British usage, a type of tournament or competition in which a loser is eliminated at the end of each round;
e.g. The tennis tournament is a knockout, where losing one match can remove you from the competition.
(adjective)
having a part of or the entirety of a gene removed or made inactive by genetic engineering, used to describe organisms;
e.g. The experiment was carried on knockout mice that were missing the gene of interest.