A phaser array is an arrangement of phaser emitters located in strategic areas of a starship, space station, or other vessel. In the 23rd century, phaser arrays had smaller numbers of emitters, and were commonly referred to as phaser banks. (TOS: "The Corbomite Maneuver", "Balance of Terror") By the 24th century, Starfleet began to fit its vessels and facilities with phaser arrays that contained much larger numbers of emitters.
During the firing sequence of some shipmounted arrays, phaser energies would travel along the emitter strip and converge at the best position for targeting. (Star Trek: The Next Generation)
Galaxy-class ships, in the docked configuration, generally exhibited eleven phaser arrays. Two large arrays encircled the dorsal and ventral surfaces of the command module. The other nine arrays were on the engineering module: two upper aft arrays inward of the saucer impulse engines, one on the ventral surface of the engineering section, two lateral arrays on the warp nacelle pylons, and two aft arrays each on the dorsal and ventral pylon surfaces proximate to the aft photon torpedo launcher. An additional array, on the "cobra head" of the engineering module, was only exposed and usable when the ship was separated into its two components. (Star Trek: The Next Generation)
Intrepid-class vessels were equipped with thirteen phaser arrays. On the primary hull, there were two main arrays (one starboard, one port) on each of the dorsal and ventral surfaces, and two smaller aft arrays (also one starboard and one port) on each surface. The other five arrays were on the engineering section: one on the ventral surface, two lateral ventral arrays on the warp nacelle pylons, and two aft arrays on the underside of the lateral pylon. (Star Trek: Voyager)
The later Sovereign-class starships originally had twelve arrays, including seven dorsal arrays and five ventral arrays. (Star Trek: First Contact) By 2379, four additional arrays were added to the nacelle pylons of the Sovereign-class USS Enterprise-E, bringing the total complement of arrays to sixteen. (Star Trek Nemesis)
In 2371, Korinas asked Benjamin Sisko for information on the USS Defiant's phaser array. (DS9: "Defiant") In the early months of the Dominion War, the crew of the Defiant placed depleted phaser array power cells in the mess hall as a sign they would not stop fighting. (DS9: "Behind the Lines")
Cardassian Galor-class vessels were equipped with multiple phaser arrays which could cover 360 degrees. These were used during Operation Return primarily to destroy Federation attack fighters. (DS9: "Sacrifice of Angels")