Oracle Text

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

Card Rulings

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.