Interferometry is the practice of using a two-or-more-element radio telescope array to observe astronomical sources. The array itself, along with the electronics used to synthesise the signals detected by the telescopes, are what we call the interferometer.
A two-element interferometer consists of two telescopes seperated by a distance , called the baseline. Both antennas receive electromagnetic radiation from an astronomical source in the sky. Because astronomical sources are far away, the radiation received is in the form of plane waves.