Damage. Damage is subtracted from a creature's hit points. This has no effect on its capabilities until the creature drops to 0 hit points.

If a creature has 0 hit points, see the following: 
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  • Most monsters die instantly. See here.
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  • For a player character or special monster:
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  • If there is damage remaining that equals or exceeds its hit point maximum, it dies instantly.
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If you take damage while at 0 hit points, you suffer a death saving throw failure. If from a critical hit, you suffer two failures.

Healing a creature with 0 hit points saves it. You can also stabilize a creature so that it isn’t killed by a failed death saving throw.

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The number of successes and failures is reset to zero when you regain any hit points or become stable.