Knockout (aka “K.O.”, occ. “kayo”) occurs when someone’s lifebar is completely depleted. It is usually the most important way to win a round.
Most games take an extra note when the winner has a full lifebar when this has occurred, usually declaring “Perfect!”
Most games declare knockout by displaying “K.O.” or “knockout” on the screen and may also (or in lieu of display) declare it with a voiceover of the word.
Some games don’t declare knockout (e.g., Street Fighter II, which just says, “You win.”).
Some games have teams, declaring a knockout separately from victory by knockout (e.g., KOF XI notes, “Down!” when a team member is knocked out, but then notes “K.O.” if everyone on one team gets knocked out).
Some games don’t say anything at all (World Heroes just plays a bell sound).
Declarations for other “victory” conditions vary (e.g., “time over”, and “Double K.O.” / “Draw Game”).