McGraw Hill Dictionary of Scientific and Technical Terms 3rd Edition 1984
Sensor
The generic name for a device that senses either the absolute value or change in a physical quantity such as temperature, pressure, flow rate, or pH, or the intensity of light, sound, or radio waves and converts that change into a useful input signal for an information gathering system; a television camera is therefore a sensor and a transducer is a special type of sensor.
Transducer
Any device or element which converts an input signal into an output signal of a different form; examples include the microphone, phonograph, pickup, loudspeaker, barometer, photoelectric cell, automobile horn, doorbell, and underwater sound transducer.
Oxford English Dictionary 2nd edition CD-ROM version
Sensor
A device giving a signal for the detection or measurement of a physical property to which it responds.
Transducer
Any device by which variations in one physical quantity (e.g. pressure, brightness) are quantitatively converted into variations in another (e.g. voltage, position).
IEEE Standard Dictionary 1993 (I only re-typed the first definition here)
Sensor
Device that responds to a physical stimulus and transmits a resulting signal.
Transducer
A device that is actuated by power from one system and supplies power in any other form to another system.