LSDPronunciation: /ɛlɛsˈdiː/Translate LSD | into Italian | into Spanish Definition of LSD noun [mass noun] a synthetic crystalline compound, lysergic acid diethylamide, which is a powerful hallucinogenic drug. Chemical formula: C20H26N2O Origin: 1950s: abbreviationLSD in other Oxford dictionaries Definition of LSD in the US English dictionary