maser

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A device that generates or amplifies electromagnetic radiation, especially microwaves, of a coherent wavelength by stimulated emission.
• noun A natural object or region of space that acts like a maser, as a star-forming region of intersellar space.

from The Century Dictionary.

• noun An obsolete form of mazer.

from the GNU version of the Collaborative International Dictionary of English.

• noun Same as mazer.
• noun (Physics), acronym An electronic device producing coherent monochromatic microwave radiation; it produces less noise than other forms of microwave oscillator.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Obsolete spelling of mazer.
• noun physics a device for the coherent amplification or generation of electromagnetic radiation (especially of microwave frequency) by the use of excitation energy in resonant atomic or molecular systems
• noun astronomy Any celestial object that generates microwaves using the same method

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an acronym for microwave amplification by stimulated emission of radiation; an amplifier that works on the same principle as a laser and emits coherent microwave radiation