Damage Counter
Damage Counters are one of two types of counters that can be put on top of face-up cards on the field. Damage Counters are typically collected on creatures that receive damage (either combat or effect damage). As with other counters, damage counters are persistent as long as the card is face-up on the field; they are not cleared at end of turn or step, but can be healed by effects.
Every creature on the battlefield must have a Life that represents that creature's maximum life. If at any point in the game the number of damage counters on a creature equals or exceed their life, the creature is immediately destroyed as a state-based action (i.e. the destruction doesn't go to the stack and cannot be responded to).