Knocked Out, also known as Knock Out, K.O., or KO'd, is a common phrase and state/condition used in video games. In Dead or Alive, a character is KO'd and thus loses the round when their health bar is exhausted, or if they have less health than their opponent when the timer runs out. Sometimes after KO, a countdown starts and can lead to Game Over if the player does not restart the match again.
It should be noted that in Dead or Alive, being KO'd does not mean that the character has been literally knocked out; they are still able to talk and even move a little depending on how they land on the ground (see below).
While in the majority of DOA games an opponent can continue to be hit after they are KO'd (especially if the KO attack did not knock them over), in Dead or Alive 5 certain actions, such as a Power Blow KO, fixes the fighters in slow motion. This prevents any further attacks from taking place. Post-KO attacks have no additional consequence, merely serving as a visual flourish. Since Dead or Alive 4, fighters react realistically to attacks when knocked out, e.g. grabbing their stomachs in pain or shaking their heads. After a while, they'll actually stop moving all together, but this is most noticeable in Dead or Alive 5.
In Dead or Alive 6, characters can generally no longer be continuously hit after a KO, for unknown reasons, although it is still possible in survival mode, especially in older versions of the game where players could continuously hit the opponent and even "taunt" afterwards to prolong the opponent writhing on the ground before becoming unconscious.