Hallucination
A hallucination was 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 was not real, and that others could not see.

Causes
The Caves of No'Mat were known to induce hallucinations in Klingons by way of heat and vapors. ()

The tidal harmonies on Balosnee VI could cause hallucinations. ()

The psychotropic drug tropolisine caused hallucinations. ()

The telepathic pitcher plant, a large bioplasmic organism, lured starships into its maw by creating a neurogenic field that showed their crews hallucinations of their desires. ()

The Native American vision quest ritual induced a hallucinogenic state. (; )

The Botha of the Delta Quadrant attacked others by inducing hallucinations through a psionic field, eventually trapping each individual's mind in a fantasy world. ()

Denobulans considered hallucinations healthy, particularly for persons under stress, as they saw it as a harmless way to release nervous energy. ()

Inhalation of nitrous oxide caused hallucinations in Humans. ()

Schizophrenia and head trauma caused hallucinations in some cases. ()

Incidences
In 2152, Ensign Hoshi Sato experienced a slightly extended time in a pattern buffer due to an ion storm, resulting in an elaborate hallucination involving her becoming out of phase and unable to interact with anything or anyone else, alien saboteurs, and a fictitious story of a similar transporter accident happening to a Cyrus Ramsey. ()

In 2267, Doctor McCoy speculated that seeing individuals who seemed to be the fictional characters  and the White Rabbit may have been a hallucination. When Spock later asked Captain Kirk if these and other strange sights might have been hallucinations, Kirk replied that one of the oddly appearing individuals, a former acquaintance named Finnegan, had painfully struck him on the jaw, suggesting these were not hallucinations. Thereafter, however, McCoy tried to reassure Yeoman Tonia Barrows that they were and that hallucinations were incapable of doing them any harm. As it turned out, the individuals were actually robots. ()

In 2367, when the  was trapped in a Tyken's Rift, the crew experienced prolonged REM sleep deprivation and many suffered disturbing hallucinations as a result. ()

In 2374, while traveling through a Mutara class nebula with dangerous radiation, Seven of Nine experienced a number of hallucinations. These included the warp drive turning green, burning crewmen, a distressed Tom Paris, and elaborate scenarios involving an alien called Trajis Lo-Tarik. ()

In 2369, Doctor Julian Bashir believed he was hallucinating when he saw the affectionate Jadzia Dax on his bed. ()

In 2382, Beckett Mariner and Bradward Boimler were exposed to an environment rich in nitrous oxide intended for a Tamarian defrin root, resulting in vivid hallucinations. D'Vana Tendi, an Orion immune to the hallucinogenic properties, was forced to drag them out of the hydroponics bay into breathable air. ()

In 2399, Dahj Asha became concerned by the fact that she was experiencing auditory hallucinations, thinking it might be caused by either schizophrenia or head trauma. ()

See also
 Orb shadow

External link
 
de:Halluzination
Medical conditions