Noun
English Wikipedia has an article on:electretWikipedia
electret (plural electrets)
(physics) A solid dielectric having a quasi-permanent charge; usually a metallized film; used in electroacoustic and electromechanical transducers and in air filters.
Electret microphones and condenser microphones employ electrostatics—as the sound wave strikes the microphone's diaphragm, it moves and induces a voltage change. Source: Internet