An electret microphone is a type of condenser mic that eliminates the need for polarizing power for the capacitor by using permanently charged dielectric material. The name electret is an amalgam of electrostatic and magnet. The electret's dielectric material has a more-or-less permanently embedded static electric charge that lasts well-beyond the life of the microphone.
Dielectric material was first proposed for condenser mics back in the 1920s, but weren't really practical until Bell Labs invented the thin metallized electret Teflon foil in 1962.
There are three types of electret mics. Some use dielectric material for the back plate (the non-moving plate) of the capacitor capsule in a condenser mic. Others use a front electret design where the back plate is eliminated and the sides of the capsule are coated with electret material. The capacitor is formed by the diaphragm and the inside surface of the capsule. A third type actually uses the electret material for the capacitor's flexible diaphragm. Although this is the most common type of electret mic, it's not often the best quality, since the electret material doesn't make the best diaphragm.
Even though electret mics don't require additional polarizing voltage, they usually have an integrated preamp that does require power. This power is often supplied by an internal 1.5 volt battery, like a standard AA that is often left always connected, since the electret drain on the battery is extremely low. Phantom power can also supply that voltage.
