From 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]