The final, and least common, solar radio emission mechanism is electron-cyclotron maser emission (ECME). Maser is an acronym for "microwave amplification by stimulated emission of radiation", which originally referred to a laboratory device that can produce intense radiation of a specific frequency through stimulated emission. Stimulated emission is a process by which a group of atoms are moved into higher energy levels (above thermal equilibrium) and then stimulated to release that extra energy all at once. Such population inversions can occur naturally to produce astrophysical masers, which are sources of very intense radiation of specific spectral lines.