What is the difference between a sensor and a transducer?

A sensor detects and responds to a physical quantity (temperature, pressure, light) but may not produce a standardized output. A transducer converts one form of energy to another - specifically converting the sensed quantity into an electrical signal suitable for measurement. In practice, the terms are often used interchangeably, but technically a transducer includes the conversion element. For example, a thermocouple is both a sensor (senses temperature) and transducer (outputs voltage).