Wordnet 3.0
NOUN (1)
1. 
 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; 
The Collaborative International Dictionary of English v.0.48:
maser \ma"ser\, n.
   1. Same as Mazer.
      [1913 Webster]
   2. (Physics) [from Microwave Amplification by Stimulated
      Emission of Radiation.] An electronic device producing
      coherent monochromatic microwave radiation; it produces
      less noise than other forms of microwave oscillator.
      [acronym]
      [PJC] MASH
WordNet (r) 3.0 (2006):
maser
    n 1: 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