Powers limited in use to the battle/scenario itself. Typically recharges once the encounter/scenario for normal use again.
Used in a power limited system that typically has at-will and daily powers. Encounter powers are typically more powerful than at-will powers (and less than daily powers) and more readily available than daily powers (but not as readily available as at-will powers). Game systems of note that use the power limited system: Dungeon and Dragons 4e and Gamma World