Pronunciation: (dub'ul ak'ting),[key] —adj. 1. (of a reciprocating engine, pump, etc.) having pistons accomplishing work in both directions, fluid being admitted alternately to opposite ends of the cylinders. Cf. single-acting. 2. (of a hinge of a door or the like) permitting movement in either direction from the normal, or closed, position.
3. having twice the usual effectiveness, strength, use, etc.