Maser, which began as an acronym for Microwave Amplification by Stimulated Emission of Radiation is a device (or situation) where atoms are excited and incoming radiation at a specific Wavelength causes Emission by the atoms of more radiation at the same wavelength (Maser Emission). The potential for such stimulation was described by Einstein in 1917 and such devices amplifying Microwaves were developed in the 1950s.
Natural masers can occur in space in Molecular Clouds including water or other molecules (OH, CH3OH, CH2O, SiO). Water molecules excited in Star-Forming Regions emit 22.0 GHz radiation. Very powerful masers also occur in Active Galactic Nuclei.