Something that changes one form of energy into another, Sensors or actuators can be called tranducers. Transducers are device that can perform an input (Sensors) or an output (Actuators) function. For example a force sensor converts the physical change into an electrical signal, an actuator transforms an electrical signal into a physical change like movement or sound. Another example of transducers are the microphone that converts sound waves into electrical signal, or a loudspeaker that converts this electrical signal back into sound waves.