Hallucinogen: A drug that causes hallucinations (profound distortions in a person's perceptions of reality). Under the influence of hallucinogens, people see images, hear sounds, and feel sensations that seem real but do not exist. Some hallucinogens also produce rapid, intense emotional swings. Hallucinogens cause their effects by disrupting the interaction of nerve cells and the neurotransmitter serotonin. Distributed throughout the brain and spinal cord, the serotonin system is involved in the control of behavioral, perceptual, and regulatory systems, including mood, hunger, body temperature, sexual behavior, muscle control, and sensory perception.
LSD (an abbreviation of the German words for "lysergic acid diethylamide") is the drug most commonly identified with the term "hallucinogen" and the most widely used in this class of drugs. It is considered the typical hallucinogen, and the characteristics of its action and effects apply to the other hallucinogens, including mescaline, psilocybin, and ibogaine.