interferometer: Definition and Much More from Answers.com

n.

Any of several optical, acoustic, or radio frequency instruments that use interference phenomena between a reference wave and an experimental wave or between two parts of an experimental wave to determine wavelengths and wave velocities, measure very small distances and thicknesses, and calculate indices of refraction.

interferometric in'ter·fer'o·met'ric (-fîr'ə-mĕt'rĭk) adj.
interferometrically in'ter·fer'o·met'ri·cal·ly adv.
interferometry in'ter·fe·rom'e·try n.