An activated ability is an ability that can be played 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 (
). 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 play its activated abilities unless the ability says otherwise. Also, you can usually play an activated ability any time you have priority, unless it says otherwise. Excpet for those that are mana abilities, they use the stack.
A player cannot play an activated ability of a creature that includes the tap symbol () in its cost unless he or she has controlled the creature continuously since the start of his or her 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.
Examples
Below are examples of different activated abilities.
Lore Broker's activated ability can be activated any time the controlling player could play an instant, which means he or she must have priority to play it. The cost includes the tap symbol , 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 his or her turn. (This is informally known as "Summoning Sickness".)
Nezumi Bone-Reader's activated ability's cost does not include the symbol, which means it can be used even if it has summoning sickness. It can however only be used in the controller's main phases when he or she has priority and the stack is empty.
Because Orochi Leafcaller's activated ability produces mana, it is a mana ability, and thus it doesn't use the stack. This means the controller doesn't lose priority when he or she plays this ability, and that in turn means that an opponent cannot respond to this activation.
Squallmonger's ability can be activated by other players than the controller, because the card explicitly says so. It still requires a player to have priority though.
Because Nullmage Shephard's ability's cost doesn't include the symbol, the controller is allowed to tap the Nullmage Shephard, or other creatures, with summoning sickness.
Ghost-Lit Raider has two activated abilities. The card doesn't explicitly say when you can play the two different abilities, but it is inferred by the cost.
The first requires the controller to tap it, an thus requires the card to be in play, as only permanents can be tapped. The second, which incidentally has the ability word "Channel", requires the controller to discard it from his or her hand. This implies that it can only be played when it is in its controllers hand.
Shard Phoenix also has two activated abilities. The first requires Shard Phoenix to be in play, and for its controller to have priority. The second ability can only be played when the card is in its owner's graveyard and only during its owners upkeep step.
Sekki, Seasons' Guide has one activated ability which can only be played when the card is in a graveyard.
Formal rules
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