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.