LSD researcher Humphrey Osmond coined the term psychedelic in 1956, in a letter to Aldous Huxley. It is from the greek roots psyche, meaning mind or soul, and delos, meaning revealing or manifesting. It was coined as a reaction against the terms hallucinogenic (causing hallucinations) and psychotomimetic (mimicking psychosis), which Osmond felt were both loaded and inaccurate when applied to drugs like LSD, mescaline and psilocybin.

To fathom Hell or soar angelic,
Just take a pinch of psychedelic

- from Osmond's letter to Huxley.

See the Essential Psychedelic Guide and the psychedelic drugs node for more on the psychedelics.