An interferometer is a device which combines electromagnetic waves in such manner that their interference can be measured to reveal properties of waves. Astronomy often uses the term for radio interferometers that consists of two or more individual radio telescopes, each of which translates received radio waves into electronic signal oscillations, which, in turn, are combined, producing analogous interference. Through analysis of the interference patterns, the direction of the source of a signal can be determined with very high precision (down to microarcseconds), yielding a very precise location in the celestial sphere, and by observing adjacent locations, yielding detail. A sophisticated type, typically using a number of radio telescopes, carries out what is termed aperture synthesis, producing an image of the radio source. This is now carried out at the short-wavelength end of radio (around where microwave radio meets infrared, about a millimeter wavelength), producing probably the most detailed images of astronomical objects.
Astronomy also has optical interferometers, doing the analogous processing at visible light wavelengths, but the corresponding frequency is too high for the electronic processing used to measure the interference of radio waves and the interference must be carried out within optics, requiring extremely precise physical positioning of elements of the interferometer necessary to deal with the wavelengths as short as the typical width of a molecule. Such interferometers are useful but the challenges of making them work has prevented more wide-spread use.
A type of interferometer, the Michelson interferometer is used within some optical astronomical instruments and also is the heart of gravitational-wave detectors such as LIGO.
Many spectroscopes use wave interference produced by a grating, thus are technically interferometers but invariably go by the name spectroscope, indicating the instrument's function.