Activated ability

From CMC Wiki

An activated ability is an ability of an entity that requires purposeful usage of the ability, rather than happening automatically. These are designated on the card with an arrow followed by the ability's costs and effects. To use an activated ability, click on the "Ability" button, then click on the entity whose ability you would like to use; some abilities will then require designating a target afterwards.

There are some card abilities, such as that of Graviton Sword, that can only target monsters with activated abilities. Similarly, Fleshbomb and Sentry Statue can only target monsters without one. The presence of an activated ability is distinguished by an ability speed letter on the corner of a card. If a card loses its activated ability, the speed letter will vanish to identify its lack of ability.

Some examples of cards with activated abilities include Babble, The Nothing, and TR-32 Variable.