A knockout (abbreviated as "K.O") is when a player destroys another player's character by bringing their health bar to zero. Some games contain weapons, or gear, which most of them are for fighting players. For example, you would probably find weapons in a swordfighting game (KOs are Knockouts, are usually 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.
In 2006, KO's would not register to the ROBLOX user's profile.
Recently, players have been using their own gear items to kill others in many places that allowed gear (which is considered griefing), which eventually led to the retirement of the Combat Initiation, Warrior, and Bloxxer badges.
Knockouts no longer exist as a stat in ROBLOX, as they were removed on July 8, 2015. All three badges associated with knockouts have been discontinued, and the forum post can be found here.