Stifle

Instant    (CMC:1)

Counter target activated or triggered ability. (Mana abilities can't be targeted.)

"Superior force meets woefully inadequate object. What paradox were you expecting?"

108 CNS • ENEric Fortune

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Legal in: Onslaught Block,Legacy,Vintage,Commander

Oracle Text (click to copy):

10/4/2004
An activated ability has a "Cost: Effect" format. Look for the colon. A triggered ability starts with "when", "whenever", or "at".
10/4/2004
Turn-based actions and special actions like the normal card draw, combat damage, or turning a face-down creature face up can't be targeted.
10/4/2004
It can target delayed triggered abilities. For example, a card that says "Whenever a player cycles a card, you may exile target creature. If you do, return that creature to the battlefield at the beginning of the end step." triggers when a player cycles a card and creates a delayed trigger that happens at the beginning of the end step. You can choose to target the "at the beginning of the end step" trigger when it is placed on the stack at end of turn.