Reinforce

Reinforce is an activated ability that functions only while the card with reinforce is in a player’s hand. It was introduced in Morningtide.

From the Comprehensive Rules:

502.77. Reinforce
- 502.77a 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.”
- 502.77b Although the reinforce ability is playable only if the card is in a player’s hand, it continues to exist while the object is in play and in all other zones. Therefore objects with reinforce will be affected by effects that depend on objects having one or more activated abilities.