Plain English Meaning
A hydraulic ram that is pushed by fluid pressure both ways. Pressure on one side moves it out, pressure on the other side moves it back in.
Full Definition
A hydraulic actuator in which fluid pressure can be applied to either side of the piston, allowing the servo to produce powered movement in both directions. Hydraulic fluid is directed to one side to extend the piston and to the other side to retract it, with the opposite side venting to return.
Context Anchor
Seen in hydraulic system discussions, especially where pressurized fluid is used to move or position aircraft parts such as landing gear, brakes, flaps, or control surfaces.