is an electronic device that converts energy from one form to another.
Common examples include microphones, loudspeakers, thermometers, position and pressure sensors, and antenna.
Although not generally thought of as transducers, photocells, LEDs (light-emitting diodes), and even common light bulbs are transducers.
Definition from WhatIS.com
A transducer is a device, usually electrical, electronic, electro-mechanical, electromagnetic, photonic, or photovoltaic that converts one type of energy or physical attribute to another for various purposes including measurement or information transfer (for example, pressure sensors).
The term transducer is commonly used in two senses; the sensor, used to detect a parameter in one form and report it in another (usually an electrical or digital signal), and an actuator may be described as opposite to a sensor-it converts electrical signal into generally nonelectrical energy. An example of a transducer is a loudspeaker which converts an electrical signal into a variable magnetic field and, subsequently, into acoustic waves.