KO

A KO, short for Knock-Out or Knock-Off, 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. The KO'd player is sent to a revival platform to re-enter the match. The following may then happen depending on the type of match:
 * In a Time match, the KO'd player will lose a point, while the player who made the KO gains a point.
 * In a Stock match, the KO'd player loses a stock.
 * In a Coin match, the KO'd player loses half their coins (if the number is odd, the "smaller half" is lost). SSBB prevents a player from losing more than 100 coins at once. The lost coinage will fly into play if the KO was made off the side.
 * In the Subspace Emissary, the next character in the lineup is loaded and teleported into play without a revival platform. One stock is also lost.
 * Characters can also be KO'd in the Super Smash Bros Brawl's Subspace Emissary if they are crushed by moving walls, ceilings, or floors.

If a player has no stock remaining, then they do not reappear. They are then either removed from play (multiplayer matches) or receive a Game Over (single player modes).

Who made the KO?
In Super Smash Bros. and Super Smash Bros. Melee, each character has a "KO property" which starts out empty. When a character is hit by an attack (be it a physical attack, a projectile, an item, etc), their KO property is set to match whoever made the hit. Stage elements do not change the KO property. The KO property is reset once a character lands on the ground and is no longer moving as a result of the attack's knockback. When the character is KO'd, the KO goes to whoever matches the KO property. If the property is null, the KO counts as a self-destruct. In simple terms, it goes to whoever made the last hit.

However, for whatever reason, this format was heavily altered for Super Smash Bros. Brawl. In SSBB, a character's KO property is never reset during a stock. If a character is hit at any time during a stock, it becomes impossible to self-destruct; a KO will be given to whoever make the last hit. So if a character is hit by an attack and jumps off the edge, it will count as a KO, not an SD. This change is baffling to say the least; it gives other characters free KOs when the target character dies on his/her own.

Also see

 * Self-destruct
 * Star KO
 * One-hit KO