Interferometer

noun

1. 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.
2. An instrument for the measurement of lengths by means of the phenomena resulting from the interference of two rays of light.
3. 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.
4. any of several instruments that use the interference of waves to determine wavelengths and wave velocities, determine refractive indices measure small distances, temperature changes, stresses, and many other useful measurements
5. any measuring instrument that uses interference patterns to make accurate measurements of waves