A holographic projector (also called a holographic emitter or holo-emitter for short) was a device used to project holograms.
History and specifics
In 2270, Montgomery Scott programmed a holographic projector to provide images for a Starfleet initiation for starship crew completing their first tour of space duty. However, Klingons using Talosian illusion-creating techniques overrode the projector, creating vivid Carnellian acid-snakes to threaten Pavel Chekov. (TOS comic: "The Final Voyage")