The Collaborative International Dictionary of English v.0.48:

laser \laser\ n. [acronym from Light Amplification by Stimulated
   Emission of Radiation: ca. 1955.]
   A device which produces an intense, usually narrow,
   monochromatic beam of coherent light; called also optical
   maser. It has various forms, and is used in various
   applications in science and technology. The beam is produced
   by boosting the majority of the electrons in specific
   radiation-absorbing atoms in a medium into a higher energy
   level, from which they are stimulated by their own emitted
   radiation to drop back synchronously to their lower energy
   level, and emit light which is in phase (coherent).

   Syn: optical maser.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

optical maser
    n 1: an acronym for light amplification by stimulated emission
         of radiation; an optical device that produces an intense
         monochromatic beam of coherent light [syn: laser,
         optical maser]