Lysergic acid diethylamide (LSD-25, LSD), formerly lysergide, commonly known as acid, is a semisynthetic psychedelic drug of the ergoline and tryptamine families. LSD is non-addictive, non-toxic, and is well known for its psychological effects which can include closed and open eye visuals, synaesthesia, a sense of time distortion, ego death and profound spiritual experiences, as well as for its key role in 1960s counterculture. It is used mainly by psychonauts as an entheogen and in psychedelic therapy.

LSD was first synthesized by Albert Hofmann in 1938 from ergot, a grain fungus that typically grows on rye. The short form LSD comes from its early code name LSD-25, which is an abbreviation for the German "Lysergsäure-diethylamid" followed by a sequential number. LSD is sensitive to oxygen, ultraviolet light, and chlorine, especially in solution, though its potency may last for years if it is stored away from light and moisture at low temperature. In pure form it is a colourless, odourless, and mildly bitter solid. LSD is typically delivered orally, usually on a substrate such as absorbent blotter paper, a sugar cube, or gelatin. In its liquid form, it can be administered by intramuscular or intravenous injection. The threshold dosage level needed to cause a psychoactive effect on humans is between 20 and 30 µg (micrograms).

Introduced by Sandoz Laboratories, with trade-name Delysid, as a drug with various psychiatric uses in 1947, LSD quickly became a therapeutic agent that appeared to show great promise. However, the emerging recreational use of the drug by youth culture in the Western world during the 1960s led to a political firestorm that resulted in its prohibition. A number of organizations—including the Beckley Foundation, MAPS, Heffter Research Institute and the Albert Hofmann Foundation—exist to fund, encourage and coordinate research into its medicinal uses.