Hallucination

A hallucination was the perception of something that was not present.

Hallucinations were a side effect of the Klingons' thought-shield, especially to James T. Kirk and Montgomery Scott when they visited Organia (which was confined in one) in 2269. 

In 2326, Cadet Jean-Luc Picard at first thought his sightings of Delcara were hallucinations. 

In 2369, Doctor Beverly Crusher discovered that Pelavians exhaled chemicals which caused hallucinations in Humans when inhaled. (SS TNG (On the Scent of Trouble))