A hologenerator was a device used to create holographic imagery. It was similar in function to holodecks, holosuites, and the EMH mobile emitter. Some devices of this sort used omicron particles to create holograms. (DS9: "Shadowplay") Holograms projected by Federation hologenerators could be interfered with by Borg lasers (Star Trek: First Contact).
Federation anthropological teams used hologenerators in field studies of pre-warp civilizations. (TNG: "Who Watches The Watchers"; Star Trek: Insurrection)