A bare piezoelectric transducer that can be used either as a sensor, or as a speaker.
Good to have a few of these around.
A handy component for all kinds of projects, as they can both produce sound and revive it. Not that all speakers and mics cant, but as far as cheep components go they are rather versatile.
MCM filled my order promptly and delivery was fast. The item I ordered was top quality as is everything I have ordered from them.