Activated ability

An activated ability is an ability that can be activated by a player by paying a cost. The cost can be almost anything, such as sacrificing a permanent, paying mana, paying life, or tapping the permanent with the ability (undefined). Activated abilities are usually found on permanents, but can sometimes be used on cards in other zones, such as the graveyard or a player's hand.

Only an object's controller (or owner, if it has no controller) can activate its activated abilities unless the ability says otherwise. Also, you can usually activate an activated ability any time you have priority, unless it says otherwise. Except for those that are mana abilities, activated abilities use the stack.

A player cannot activate an activated ability of a creature that includes the tap symbol (undefined) or the untap symbol in its cost unless they have controlled the creature continuously since the start of their most recent turn. This condition is informally known as "summoning sickness".

Activated abilities cannot be countered by spells or abilities that counter spells, because they aren't spells. However, there are cards, such as Stifle, Squelch, and Voidslime, that can be used to counter these abilities.

Most activated abilities are written in the form "[Cost]: [Effect]". Almost every activated ability has a colon separating its cost from its effect, and colons aren't used in Magic rules text for anything other than activated abilities. The colon is the one punctuation mark that has a rules meaning in the Magic game. An exception to this are loyalty abilities of planeswalker s which are also activated abilities and which use a different notation.

Rules
undefined undefined undefined

Examples
Below are examples of different activated abilities.

Lore Broker's activated ability can be activated any time the controlling player could cast an instant, which means they must have priority to activate it. The cost includes the tap symbol undefined, which means that because Lore Broker is a creature, the ability cannot be played if the controller has not controlled it from the beginning of their turn. (This is informally known as "summoning sickness".)

Nezumi Bone-Reader's activated ability's cost does not include the undefined symbol, which means it can be activated even if it has summoning sickness. However, because it can only be activated when the controller could cast a sorcery, it can only be activated during the controller's main phase when they have priority and the stack is empty.

Squallmonger's ability can be activated by other players than the controller, because the card explicitly says so. The player activating the ability must still have priority in order to do so, however.

Ghost-Lit Raider has two activated abilities. The card doesn't explicitly say when you can activate the two different abilities, but it is inferred by the cost.

The first requires the controller to tap it, and thus requires the card to be on the battlefield, as only permanents can be tapped. The second (which has the ability word Channel) requires the controller to discard it from their hand. This implies that it can only be activated when it is in its controller's hand.