An actuator is a component of a machine that produces force, torque, or displacement, when an electrical, pneumatic or hydraulic input is supplied to it in a system (called an actuating system).
Actuators are mechanical or electro-mechanical devices that generate controlled motion or positioning, operating through electrical, manual, pneumatic, or hydraulic power sources. They primarily produce two types of motion: linear and rotary.
Actuators receive a source of energy and use it to move something. To put it another way, the actuator converts a source of energy into a physical-mechanical motion. A butterfly valve is operated by turning the Handwheel connected to the rotary actuator.
Defined simply, an actuator is a device that converts energy, which may be electrical, hydraulic, or pneumatic, to mechanical output in such a way that it can be controlled. The quantity and the nature of input depend on the kind of energy to be converted and the function of the actuator.