A specific type of dynamic (as opposed to condenser) microphone design. Moving coil microphones are among the most commonly used in music and sound production. The ubiquitous SM-58 and SM-57 mics are examples of moving coil design. These mics work on very simple principles. In fact they work just like a speaker in reverse. The diaphragm has a coil of wire attached to its base. This coil is inserted into a magnetic gap. When changes in air pressure cause the diaphragm to vibrate in and out of the magnetic gap it generates an alternating current in the wire that represents the signal.
Moving Coil is also one method used in making phonograph cartridges. Moving coil designs were all but replaced by moving magnet designs (same principle, but the magnet moves instead) in the 1970′s. Moving coil phonograph cartridges have very low output (requiring a different preamp) and are very expensive compared to their moving magnet counterparts, but there are some sonic advantages to them including lower distortion and better frequency response.