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.
In addition to the colorful KO blast, nearly every fighter in Super Smash Flash 2 makes a loud, distinct vocal sound upon being KO'd, though there are some exceptions who are silent due to lacking voice lines. These exceptions are Samus, Bandana Dee, Mr. Game & Watch, Chibi-Robo, Mega Man, PAC-MAN, Bomberman, Black Mage, and Sandbag.