Servo
An electrical or hydraulic actuator connected into a flight control system. A small force on the flight deck control is amplified by the servo and provides a large force to move the control surface.
Aviation Maintenance Technician Handbook — Airframe, p. G-30
Also defined in 1 more source
A motor or other form of actuator which receives a small signal from the control device and exerts a large force to accomplish the desired work.