Hallucination

A hallucination, in broadest sense of the word, is a perception in the absence of a stimulus. In a stricter sense, hallucinations are defined as perceptions in a conscious and awake state in the absence of external stimuli which have qualities of real perception, in that they are vivid, substantial, and located in external objective space.

There are many real and fictional mind-altering drugs that can cause a person to hallucinate. The most common hallucinagenic drug is Lysergic acid diethylamide, commonly known as LSD (or LDS as James T. Kirk calls it ).

In fiction
In the pilot episode of The Twilight Zone'' entitled "Where Is Everybody?", astronaut Mike Ferris is put through a training exercise designed to prepare him for a prolonged space journey to the moon. Due to prolonged sensory deprivation, Ferris begins to hallucinate wherein he imagines himself in a quasi-dystopic future where he is seemingly the only man left on Earth.

In the 1970 film Beneath the Planet of the Apes, a clan of subterranean mutants have developed their mental faculties to the point that they can generate illusions inside the minds of others. They first used this effect in an effort to deter the advance of astronaut George Taylor by creating a wall of flame to bar his path. They later used their abilities to create apocalyptic imagery in the hopes of diverting the path of an ape war party.