The result of a game element’s use. The damage and conditions caused by an attack power are the power’s effects, for instance. Some powers have “Effect” entries, which contain some but not necessarily all of the powers’ effects. In an attack power, the effects of such an entry are not contingent on a hit or a miss.
Published in Player's Handbook 2, page(s) 219, Rules Compendium, page(s) 311.