A hallucination is a psychological term referring to visual, auditory, or some other combination of senses giving the brain unreal information, essentially causing a being to perceive something that is not real and that others cannot see.
The Caves of No'Mat are known induce hallucinations in Klingons by way of heat and cave vapors. (TNG: "Birthright, Part I")
The tidal harmonies on Balosnee VI can cause hallucinations. (DS9: "The Nagus")
The psychotropic drug tropolisine causes hallucination. (ENT: "Strange New World")
Denobulans consider hallucinations healthy, as they see it as a harmless way to release nervous energy. (ENT: "Exile", "Doctor's Orders")
The Native American vision quest ritual induces a hallucinogenic state. (TNG: "Journey's End"; VOY: "The Fight")
The Botha of the Delta Quadrant attack others by inducing hallucinations through a psionic field, eventually trapping each individual's mind in a fantasy world. (VOY: "Persistence of Vision")
The telepathic pitcher plant, a large bioplasmic organism, lures starships into its maw by creating a neurogenic field, that shows their crews hallucinations of their desires. (VOY: "Bliss")