The application of optical, infrared, or microwave radiation of appropriate frequency to a laser or maser medium so that absorption of the radiation increases the population of atoms or molecules in higher energy states.

A maser is a device for the creation, amplification, and transmission of an intense, highly focused beam of high-frequency radio waves.

The name maser is an acronym for "microwave amplification by stimulated emission of radiation", microwaves being radio waves of short wavelength, or high frequency.