The Collaborative International Dictionary of English v.0.48:

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; -- formerly also called also refractometer, but that
   word now has a different meaning.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

interferometer
    n 1: any measuring instrument that uses interference patterns to
         make accurate measurements of waves