Historical Rules · 23 April 2010
Rule 602.1
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).]" The activation cost is everything before the colon (:). An ability's activation cost must be paid by the player who is activating it. Example: The activation cost of an ability that reads "{2}, {T}: You gain 1 life" is two mana of any type plus tapping the permanent that has the ability.