It would be the electrical mechanical device, built with coiled wire, permanent magnet, and a light weight diaphragm usually paper. This turns the electrical signals to sound by vibrating the diaphragm to compress the air arounds it. I have also heard it referred to as a motor, and they are part of a speaker system.
A 3-way speaker would have 3 drivers, each designed to output different frequency ranges. Tweeters, Mids, and Subs to name a few