of or noting a mental state characterized by a profound sense of intensified sensory perception, sometimes accompanied by severe perceptual distortion and hallucinations and by extreme feelings of either euphoria or despair.
of, relating to, or noting any of various drugs producing this state, as LSD, mescaline, or psilocybin.
resembling, characteristic of, or reproducing images, sounds, or the like, experienced while in such a state: psychedelic painting.
occasionally psychodelic, 1956, of drugs, suggested by British-born Canadian psychiatrist Humphry Osmond (1917-2004) in a letter to Aldous Huxley and used by Osmond in a scientific paper published the next year; from Greek psykhe- "mind" (see psyche) + deloun "make visible, reveal," from delos "visible, clear," from PIE root *dyeu- "to shine" (see diurnal). In popular use from 1965 with reference to anything producing effects similar to that of a psychedelic drug or enhancing the effects of such a drug. As a noun from 1956.