A psychedelic experience is characterized by the perception of aspects of one's mind previously unknown, or by the creative exuberance of the mind liberated from its ordinary fetters. Psychedelic states are one of the stations on the spectrum of experiences elicited by psychedelic substances. On that same spectrum will be found hallucinations, changes of perception, synaesthesia, altered states of awareness, mystical states, and occasionally states resembling psychosis.

The term was first coined as a noun in 1956 by psychiatrist Humphry Osmond, as an alternative descriptor for hallucinogenic drugs in the context of psychedelic psychotherapy. The term featured in a now-famous exchange with Aldous Huxley, in which the ill-fated term phanerothyme was suggested:

To make this trivial world sublime,

take half a gram of phanerothyme.

Osmond responded:

To fathom Hell or soar angelic,

just take a pinch of psychedelic.

Timothy Leary, who was largely responsible for the popularization of the term "psychedelic", was a well known proponent of their use, as was Aldous Huxley. The word psychedelic should actually be spelled psychodelic, in accordance with proper Greek, as Huxley had pointed out to Osmund upon the term's conception. However, Leary thought that psychedelic sounded better and deliberately disregarded the proper spelling and pronunciation. The use of psychedelic drugs became widespread in the mid-1960s. The fashion for psychedelic drugs gave its name to the visual style of psychedelia, to a style of electronic music commonly known as psytrance and to a rock music style that became known as psychedelic music.

The impact of psychedelic drugs on western culture in the 1960s led to semantic drift in the use of the word "psychedelic", and it is now frequently applied to describe any brightly patterned or coloured object. In objection to this new meaning, and to the pejorative meanings of other synonyms such as "hallucinogen" and "psychotomimetic", the term "entheogen" was proposed and is seeing increasing use. However, many consider the term "entheogen" best reserved for religious and spiritual usage, such as certain Native American churches do with the peyote sacrament, and "psychedelic" left to describe those who are using these drugs recreationally.

At the same time as psychedelic drugs were being used by the counterculture of the 1960s, they were also being used in experiments by governments, who saw them (apparently mistakenly) as useful agents for mind control; see MKULTRA for the CIA involvement in the use of psychedelic drugs.

See also