A KO, short for knock-out or knock-off, commonly known as a death or a kill, 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:
 KO property
In Super Smash Bros. and Melee, each character has a "KO property" which starts out empty. When characters are hit by any 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 empty, the KO counts as a self-destruct. In simple terms, it goes to whoever made the last hit.
However, this format was altered for Super Smash Bros. Brawl. In Brawl, a character's KO property is only reset 3 minutes after landing on the ground. This essentially means that a character must refrain from being hit for 3 minutes for a self-destruct to actually count as such; otherwise a self-destruct will be credited as another player's KO. While this was possibly implemented to avoid griefing in online matches (by self-destructing simply to deprive other players of points), in reality its main effect is giving other players illegitimate points, as it's clear that jumping off the edge a few seconds after successfully recovering is not a KO.
There are exceptions, however, particularly during most Final Smashes. If players jump off the stage during a final smash, such as Zero Laser, then, unless someone else hit them before landing, it will count towards the user of the final smash, instead of the last one to hit them (Samus in this case). More over, dying during certain final smashes, such as Giga Bowser will result in a self-destruct instead of a KO, even if players have been damaged by opponents, previously. Characters with invincibility from a Starman will also self-destruct instead of being KO'd.
 See also