A Type-VIII phaser was a type of phaser emitter usually installed on Federation Starfleet starships.
In the 2290s, Federation starship classes such as the Excelsior-class were equipped with a number of phaser banks of this type. (Decipher RPG module: Starships)
By 2321, this type of phaser had become standard aboard Federation starships. (STA novel: Three)
By 2370, the Maquis had approriated several Federation fighters and equipped each with at least two emitters of this type. (TNG episode: "Preemptive Strike")
By the 2370s, Federation starship classes such as the Federation light cruiser (Intrepid variant) were equipped with a number of arrays of this type. (DS9 reference: Star Trek: Deep Space Nine Technical Manual)