Actuator
Definition
The muscle of a machine — a device that turns an electrical signal into physical movement, like the motor that vibrates your phone or steers a robot's arm.
Technical definition
A transducer that converts control signals (usually electrical) into mechanical motion or force. Types include DC motors, stepper motors, solenoids, piezoelectric elements, and hydraulic or pneumatic cylinders.
Example
A robot gripper closes because a small servo motor — an actuator — rotates in response to a control pulse. Your phone's vibration motor is an actuator spinning an off-center weight about 150 times per second.
Sensors and actuators are opposites that students often swap: a sensor reads the world into the machine (physical → electrical), while an actuator writes the machine's decisions back onto the world (electrical → physical). Robots need both — sensing without acting is paralysis, acting without sensing is blindness.