Knock-out
From SmashWiki, the Super Smash Bros. wiki
Although many believe "KO" is an abbreviation for Knock-Out. "KO's" are referred as Knock-Offs by the Smash Bros. instruction Booklet. A knock-Off (abbreviated "KO")is the term used to describe the situation where a player is knocked back beyond the stage's blast lines by the opponent's actions. In a time match, the player who is awarded the KO receives one point, while the KO'd player loses one. The player who is knocked out disappears from the match and returns on the revival platform, unless they have no remaining stock. The knocked-out player also loses one stock, if applicable. In the Subspace Emissary, characters will be KO'd automatically if they are crushed by solid objects such as moving walls and platforms. The revival platform is also absent; A KO'd player simply respawns.
