Source : Webster's Revised Unabridged Dictionary (1913)

Interferometer \In`ter*fe*rom"e*ter\, n. [See {Interfere} and
   -meter.] (Physics)
   An instrument for measuring small movements, distances, or
   displacements by means of the interference of two beams of
   light; -- called also {refractometer}.

Source : WordNet®

interferometer
     n : any measuring instrument that uses interference patterns to
         make accurate measurements of waves