Final answer:
A hallucinogen is a psychoactive substance that primarily alters a person's perceptions of reality.
Step-by-step explanation:
A psychoactive substance known as a hallucinogen primarily alters a person's perceptions of reality.
Hallucinogens are a class of psychoactive drugs that cause profound alterations in sensory and perceptual experiences, often resulting in vivid hallucinations. These drugs can also cause hallucinations of body sensations and a skewed perception of time.
Examples of hallucinogens include LSD and psilocybin mushrooms.