Tribute is a keyword ability.
- 702.103a Tribute is a static ability that functions as the creature with tribute is entering the battlefield. “Tribute N” means “As this creature enters the battlefield, choose an opponent. That player may have this creature enter the battlefield with an additional N +1/+1 counters on it.”
- 702.103b Objects with tribute have triggered abilities that check “if tribute wasn’t paid.” This condition is true if the opponent chosen as a result of the tribute ability didn’t have the creature enter the battlefield with +1/+1 counters as specified by the creature’s tribute ability.