Reinforce is a keyword ability.
- 702.76a Reinforce is an activated ability that functions only while the card with reinforce is in a player’s hand. “Reinforce N—[cost]” means “[Cost], Discard this card: Put N +1/+1 counters on target creature.”
- 702.76b Although the reinforce ability can be activated only if the card is in a player’s hand, it continues to exist while the object is on the battlefield and in all other zones. Therefore objects with reinforce will be affected by effects that depend on objects having one or more activated abilities.