A KO, short for Knock Out, is the term for when a character loses a stock or a point in the Super Smash Bros. series and the Super Smash Flash series. A Knock Out occurs when a character touches a stage's blast line, usually as a result of being attacked by another character or running off by themselves. In Stock matches, a character is KO'd when they lose all of their lives.
There are different types of KO's, including the Star KO and the Screen KO. A Star KO is when a character gets hit high enough past the screen and comes falling, tumbling and screaming as he/she eventually disappears in a twinkle. A Screen KO is when a character is blasted at the top of the stage and the character hits and tumbles on the screen. After a KO, the character is sent to the Revival platform. In Stock Mode, if the character is KO'd after their last stock, the game ends.