Historical Rules · 23 April 2010
Rule 112.3b
Official source text from the 23 April 2010 Rules release
Activated abilities have a cost and an effect. They are written as "[Cost]: [Effect.] [Activation restriction (if any).]" A player may activate such an ability whenever he or she has priority. Doing so puts it on the stack, where it remains until it's countered, it resolves, or it otherwise leaves the stack. See rule 602, "Activating Activated Abilities."