A Phased Array is an array of antennas where the phase of the signal is used to transmit or receive a particular Frequency to/from a particular direction. In commercial Radio, it can be used to strengthen AM signal by tuning the phasing for the carrier frequency. In astronomy, adjusting the individual timing (phase) of signals combined from multiple fixed antennas can select a specific frequency from a specific direction, making non-movable array of antennas directional. A set of fixed phased array telescopes is termed an Aperture Array.
Examples of Radio Telescope arrays that have used phased array processing:
A Phased Array Feed is an array of receivers set at the focus of a dish radio antenna, an alternative to a Horn Feed. Phased array feeds have been tried in a number of traditional radio telescopes such as Green Bank Telescope.