Interferometry is the use and analysis of superimposed Electromagnetic Radiation waves. The patterns produced by waves from slightly different directions or Wavelengths include information about relative directions and wavelengths. A device carrying out this function is called an Interferometer.
Radio Astronomy makes much use of interferometry (Radio Interferometry), using multiple Radio Telescopes (dishes) to achieve a higher Angular Resolution. Keck Observatory and other twin/multiple telescopes can be used for interferometry of Visible Light (Optical Interferometry). A type of imaging Spectrograph, the Imaging Fourier Transform Spectroscopy makes use of a type of interferometry.
Gravitational Wave detectors such as LISA also use interferometry to measure the extremely short distances involved.