A phaser array is the name given to a series of phaser emitters that are placed in strategic locations along a starships hull. While phaser banks were commonly used by Starfleet vessels in the 23rd century, by the 24th century the use of phaser arrays were standard.
Many different types of phaser arrays exist, each with different capabilities and varying levels of effectiveness. The Type-X phaser array was equipped aboard Federation starships in the 2350s and 2360s. By the 2370s, the Type-XII phaser array had been developed and installed aboard starships. (TNG reference: Star Trek: The Next Generation Technical Manual, DS9 reference: Star Trek: Deep Space Nine Technical Manual).
In the opening months of the Dominion War, the crew of the USS Defiant would place depleted power cells from the phaser array in the ship's mess hall, as a sign that they would not stop fighting until the war was over. (DS9 episode: "Behind the Lines", DS9 novelization: ...Sacrifice of Angels)