A hologenerator, also known as a holographic generator or hologram generator, was a type of matrix-processing device used to create holographic imagery which was then displayed through holo-projectors.
After the Dominion took over the planet Yadera Prime, Rurigan fled his homeworld, taking with him a hologenerator, which he used to recreate everything he had lost on a new planet. (DS9: "Shadowplay")
Federation anthropological field teams used hologram generators for their "duck blinds", used for field studies of pre-warp civilizations. The reactor used to power such a device was capable of producing 4.2 gigawatts of energy. Such a device was used in 2366 to observe the Mintakans on Mintaka III. (TNG: "Who Watches The Watchers")
After the crew of the USS Voyager transported over to the Caretaker's array in 2371, they discovered a simulation of a farm, which included the "Banjo Man" and a folk dance, Captain Kathryn Janeway had the away team, spread out and see if they could "find anything that might be a holographic generator." Harry Kim later found readings of "some kind of matrix-processing device," which suggested the presence of a holographic generator." (VOY: "Caretaker")
Later the following year, in preparation for battle with the Kazon, Voyager placed holo-projectors along the ship's hull to as to broadcast the illusion of additional ships present to aide in their defense. In doing so, however, they put a very high demand on the hologenerators before, putting into question whether an additional boost to the power input was necessary. (VOY: "Basics, Part I")