/ˈdʌb əlˈæk tɪŋ/
(of a reciprocating engine, pump, etc.) having pistons accomplishing work in both directions, fluid being admitted alternately to opposite ends of the cylinders.
(of a hinge of a door or the like) permitting movement in either direction from the normal, or closed, position.
having twice the usual effectiveness, strength, use, etc.
(of a reciprocating engine or pump) having a piston or pistons that are pressurized alternately on opposite sides
(of a hinge, door, etc) having complementary actions in opposed directions
Collins English Dictionary - Complete & Unabridged 10th Edition
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If power is added to the downward falling hammer, the hammer is referred to as double-acting.
Two steam reciprocating, double-acting, duplex pumps handle pumping duties.