The term maser, which began as an acronym for microwave amplification by stimulated emission of radiation indicates 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, or more generally, stimulated emission). The potential for such stimulated emission was described by Einstein in 1917 and such devices amplifying microwaves were developed in the 1950s. The phenomenon requires atomic excitation of a material, giving it electrons within higher electron shells than they need to be. The injection of energy into a material to create this excitation is termed "pumping".
The term maser indicates microwave frequency, but in the case of astronomical phenomena, "maser" is also used for other frequencies. In the regime normally referred to as light (visible light and much of infrared and ultraviolet), the term laser is more likely used (light amplification by stimulated emission of radiation).
Synchrotron radiation (from acceleration such as the effect of a magnetic field on relativistic electrons) also has corresponding absorption and stimulated emission and can be used to form a maser (synchrotron maser).
Natural masers can occur in space in molecular clouds including water or other molecules (OH, CH3OH, CH2O, SiO). They generate EMR via the same kind of stimulation as the electronic devices, but do not include reflecting surfaces to ramp up amplification in a specific direction. Water molecules excited in star-forming regions emit 22.0 GHz radiation. Synchrotron masers are also presumed to occur in nature. Very powerful masers also occur in active galactic nuclei. Exceptionally powerful masers are termed megamasers.