Hallucinogens, also known as psychedelics, entheogens, or historically as psychotomimetics, are a broad and diverse class of psychoactive drugs that can induce altered states of consciousness characterized by marked changes in perception, mood, cognition, and self-experience, along with a range of other psychological alterations. Hallucinogens are often categorized as either being psychedelics, dissociatives, or deliriants, but not all hallucinogens fall into these three classes.