A submission is a combat sports term for yielding to the opponent, and hence resulting in an immediate defeat.
A combat sport, or fighting sport, is a competitive contact sport with one-on-one combat.
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The submission - then also referred to as a "tap out" or "tapping out" - is often performed by clearly tapping the floor or the opponent with the hand or sometimes with the foot, to signal the opponent and possibly the referee of the submission.
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In some combative sports where the fighter has cornermen, the corner can also stop the fight by "throwing in the towel", which may count as a submission.