A mechanical, electrical, or hydraulic device, or their combination, used to effect some predetermined linear, rotary, or oscillating movement. An actuator essentially converts hydraulic or air pressure into mechanical force. Basically, there are two types of actuators: uni-directional and bi-directional. In the uni-directional actuator, the piston moves in a single direction as a result of system pressure. In a bi-directional system, the actuator’s piston moves in either direction.