Maser is the acronym for Microwave or Molecular Amplification by Stimulated Emission of Radiation, a device for amplifying microwaves by the stimulated emission of radiation.
The microwaves (stimulated emission) emitted by the maser are obtained by artificially increasing the number of atoms emitting a given frequency.
This increase is obtained by exciting a medium in a similar way to optical pumping used in lasers.
There are solid, liquid and gas masers (with reference to the amplifying medium).
Physics The ammonia maser is the reference for atomic clocks and the hydrogen maser is used in astrophysics. Masers are used in radio astronomy to amplify microwaves.