The term "psychedelic" is derived from the Ancient Greek words psych (, "soul") and dloun (, "to make visible, to reveal"), translating to "soul-revealing".
A psychedelic experience is characterized by the striking perception of aspects of one's mind previously unknown, or by the creative exuberance of the mind liberated from its ostensibly ordinary fetters. Psychedelic states are an array of experiences including changes of perception such as hallucinations, synesthesia, altered states of awareness or focused consciousness, variation in thought patterns, trance or hypnotic states, mystical states, and other mind alterations. These processes can lead some people to experience changes in mental operation defining their self-identity (whether in momentary acuity or chronic development) different enough from their previous normal state that it can excite feelings of newly formed understanding such as revelation, enlightenment, confusion, and psychosis.
Psychedelic states may be elicited by various techniques, such as meditation, sensory stimulation or deprivation, and most commonly by the use of psychedelic substances. When these psychoactive substances are used for religious, shamanic, or spiritual purposes, they are termed entheogens.
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. Seeking a name for the experience induced by LSD, Osmond contacted Aldous Huxley, a personal acquaintance and advocate for the therapeutic use of the substance. Huxley coined the term "phanerothyme," from the Greek terms for "manifest" () and "spirit" (). In a letter to Osmond, he wrote:
To make this mundane world sublime,
Take half a gram of phanerothyme
To which Osmond responded:
To fathom Hell or soar angelic,
Just take a pinch of psychedelic
It was on this term that Osmond eventually settled, because it was "clear, euphonious and uncontaminated by other associations." This mongrel spelling of the word 'psychedelic' was loathed by American ethnobotanist Richard Evans Schultes, but championed by Timothy Leary, who thought it sounded better. Due to the expanded use of the term "psychedelic" in pop culture and a perceived incorrect verbal formulation, Carl A.P. Ruck, Jeremy Bigwood, Danny Staples, Jonathan Ott, and R. Gordon Wasson proposed the term "entheogen" to describe the religious or spiritual experience produced by such substances.
From the second half of the 1950s, Beat Generation writers like William Burroughs, Jack Kerouac and Allen Ginsberg wrote about and took drugs, including cannabis and Benzedrine, raising awareness and helping to popularise their use. In the same period Lysergic acid diethylamide, better known as LSD, or "acid" (at the time a legal drug), began to be used in the US and UK as an experimental treatment, initially promoted as a potential cure for mental illness. In the early 1960s the use of LSD and other hallucinogens was advocated by proponents of the new "consciousness expansion", such as Timothy Leary, Alan Watts, Aldous Huxley and Arthur Koestler, their writings profoundly influenced the thinking of the new generation of youth. There had long been a culture of drug use among jazz and blues musicians, and use of drugs (including cannabis, peyote, mescaline and LSD) had begun to grow among folk and rock musicians, who also began to include drug references in their songs.
By the mid-1960s, the psychedelic life-style had already developed in California, and an entire subculture developed. This was particularly true in San Francisco, due in part to the first major underground LSD factory, established there by Owsley Stanley. There was also an emerging music scene of folk clubs, coffee houses and independent radio stations catering to a population of students at nearby Berkeley, and to free thinkers that had gravitated to the city. From 1964, the Merry Pranksters, a loose group that developed around novelist Ken Kesey, sponsored the Acid Tests, a series of events based around the taking of LSD (supplied by Stanley), accompanied by light shows, film projection and discordant, improvised music known as the psychedelic symphony. The Pranksters helped popularize LSD use through their road trips across America in a psychedelically-decorated school bus, which involved distributing the drug and meeting with major figures of the beat movement, and through publications about their activities such as Tom Wolfe's The Electric Kool-Aid Acid Test (1968).
Leary was a well-known proponent of the use of psychedelics, as was Aldous Huxley. However, both advanced widely different opinions on the broad use of psychedelics by state and civil society. Leary promulgated the idea of such substances as a panacea, while Huxley suggested that only the cultural and intellectual elite should partake of entheogens systematically.
In the mid-1960s the use of psychedelic drugs became widespread in modern Western culture, particularly in the United States and Britain. The movement is credited to Michael Hollingshead who arrived in America from London in 1965. He was sent to the U.S. by other members of the psychedelic movement to get their ideas exposure. The Summer of Love of 1967 and the resultant popularization of the hippie culture to the mainstream popularized psychedelia in the minds of popular culture, where it remained dominant through the 1970s. Resurgences of the style are common in the modern era.