A hallucinogen is a substance, usually a drug, that can make a person hallucinate — see or hear things that are not there, or feel that time, colour, and the self have gone strange. Some mushrooms and some laboratory drugs work this way. You meet the word in science class, in news about drugs, and in warnings. It is a rather formal, technical name; in ordinary talk people more often say “drugs that make you see things.” The word tells you what the substance does to the mind. It does not, by itself, tell you how strong it is, whether a doctor might use it, or whether the law allows it.