In telecommunication, a phased array is a group of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. This design is used in radar, and is generalized in interferometric radio antennas (usually in radio telescopes).

The relative amplitudes of--and constructive and destructive interference effects among--the signals radiated by the individual antennas determine the effective radiation pattern of the array. A phased array may be used to point a fixed radiation pattern, or to scan rapidly in azimuth or elevation.

The phased array is used for instance in optical telecommunication as a wavelength selective splitter.

Source: from Federal Standard 1037C[1] (http://glossary.its.bldrdoc.gov/fs-1037/dir-027/_3979.htm) and from MIL-STD-188

Phased arrays are required to be used by many AM broadcast stations to enhance signal coverage in the city of license, while minimizing interference to other areas. Due to the differences between daytime and nightime ionospheric propagation at AM broadcast frequencies, it is common for AM broadcast stations to change between day and night radiation patterns by switching the phase and power levels supplied to the individual antenna elements daily at sunrise and sunset.