A knockout (abbreviated as "K.O" or "KO") is when a player destroys another player's character by bringing their health bar to zero. Some games contain weapons, or gears, which are used to battle other players. For example, swords are usually found in a sword fighting game, with knockouts usually being the main objective.
Sometimes, a player can knockout another player even if they themselves have been knocked out. This can happen due to lag, or the user's weapon may be able to harm players even if the user's character is dead - for example, a rocket launcher's rockets can knockout other players even if its user is dead.
Players used their own gear items to kill others in many places that allowed gear (considered as griefing), which eventually led to the retirement of the Combat Initiation, Warrior, and Bloxxer badges.
Knockouts no longer exist as a recorded stat on Roblox, as they were removed on July 8, 2015. All three badges associated with knockouts have been discontinued, but users that previously obtained the badges will still have them shown on their profile.