Difference between Sensors and Transducers

The question "What's the difference between sensor and transducer?" comes up in the web forums once in a while.  I suspect that university professors assign this question to their students.  It prompts the students to examine the signal chain of various sensors.  It also prompts the students to examine the purpose of the sensors and transducers within larger systems.

Working definitions

A transducer is defined as a device that converts some form of energy into another.
 This definition is rooted in physics.  I saw this definition in several books (from 60s and 70s). 

A sensor is a subsystem which converts the stimulus from the physical environment into the signal interpretable by the rest of the system.
To be effective, a sensor needs to do more than just convert one form of energy into another. 
The sensor need to distinguish its target stimulus from interference.  A sensor should have an output in a form which is convenient for the rest of the system

A sensing element is a part of a sensor directly affected by the stimulus and makes initial conversion into electrical domain.  Often sensing element is a transducer (in the pure physical meaning of a term).

Discussion

A transducer can be in a role of a sensing element, or an actuator, or sometimes even both.

A good example that illustrates the practical difference between transducers and sensors is a piezo.  The piezo transducers are used in a variety of sensors measure quite different parameters (range or flow velocity, for example).  Imagine that you want to measure flow speed of air in a duct.  If you have only a transducer in your hand, you would have to spend an appreciable amount of time before you become capable of actually making a measurement.  You would have turned the piezo transducer into a flow sensor by then.

In the industry, the usage of words "sensor" and "transducer" is a matter of convention.  Some industries use the term "transducer" for both sensors and transducers.

Related

http://www.sensorland.com/GlossaryPage001.html
http://www.softnoze.com/glossary-of-automation-sensing-and-switching-terms.cfm
https://www.nap.edu/read/4782/chapter/15#107
Several glossaries for sensors terms.  Some of them have a definition for transducer.  None of them have a definition for a sensor (oddly enough).

https://groups.yahoo.com/neo/groups/sensorforum/conversations/messages/126
My earlier version of this post from 2007.