1. lysergic acid diethylamide: a highly potent hallucinogen that structurally resembles the neurotransmitter serotonin and presumably exerts its psychoactive effects by acting as a partial agonist at the 5-HT_2A serotonin receptor. It was originally synthesized from the ergot alkaloid lysergic acid in 1938 by Swiss chemist Albert Hoffman, who then discovered its hallucinogenic effects on accidental ingestion in 1943. At very low doses, it is capable of producing visual distortions (sharpened sense of color) or frank hallucinations, together with feelings of euphoria or arousal; it became a widely used and controversial recreational drug during the mid-1960s and early 1970s. The effects of LSD were the subject of research during the 1950s as a possible model for psychosis, and various attempts were made during the 1950s and 1960s to use LSD as an aid to psychotherapy (see psychedelic therapy), although they did not prove effective. The drug is usually taken orally.

2. abbreviation for least significant difference. See Fisher least significant difference test.