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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 term population inversion describes material for which the majority atoms are in such a state. The injection of energy into a material creating this state is termed "pumping".
The term maser indicates microwave frequency, but for astronomical phenomena, "maser" is commonly used for any frequency. The term laser, for light amplification by stimulated emission of radiation (indicating the analogs that produce what is called "light": visible light and much of infrared and ultraviolet), is used for artificial devices, but is less commonly used for such astrophysical phenomena.
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 within molecular clouds including masers consisting of water (i.e., a water maser) or some other molecule, such as OH, CH3OH, CH2O, or 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 (AGNs). Exceptionally powerful masers are termed megamasers.
Masers have been used to measure distances to galaxies on the order of 100 Mpc distant: VLBI can determine radial velocities (through Doppler shifts), relative peculiar velocities, and angular distances between water masers embedded in an SMBH accretion disk: this is sufficient information to work out the distance to the SMBH with very little error, as well as the mass of the SMBH.