The neologism entheogen was coined in 1979 by a group of ethnobotanists and scholars of mythology (Carl A. P. Ruck, Jeremy Bigwood, Danny Staples, Richard Evans Schultes, Jonathan Ott and R. Gordon Wasson). The term is derived from two words of Ancient Greek, ἔνθεος (éntheos) and γενέσθαι (genésthai). The adjective entheos translates to English as "full of the god, inspired, possessed", and is the root of the English word "enthusiasm." The Greeks used it as a term of praise for poets and other artists. Genesthai means "to come into being." Thus, an entheogen is a drug that causes one to become inspired or to experience feelings of inspiration, often in a religious or "spiritual" manner.
Entheogen was coined as a replacement for the terms hallucinogen and psychedelic. Hallucinogen was popularized by Aldous Huxley's experiences with mescaline, which were published as The Doors of Perception in 1954. Psychedelic, in contrast, is a Greek neologism for "mind manifest", and was coined by psychiatrist Humphry Osmond; Huxley was a volunteer in experiments Osmond was conducting on mescaline.
Ruck et al. argued that the term hallucinogen was inappropriate owing to its etymological relationship to words relating to delirium and insanity. The term psychedelic was also seen as problematic, owing to the similarity in sound to words pertaining to psychosis and also due to the fact that it had become irreversibly associated with various connotations of 1960s pop culture. In modern usage entheogen may be used synonymously with these terms, or it may be chosen to contrast with recreational use of the same drugs. The meanings of the term entheogen were formally defined by Ruck et al.:
In a strict sense, only those vision-producing drugs that can be shown to have figured in shamanic or religious rites would be designated entheogens, but in a looser sense, the term could also be applied to other drugs, both natural and artificial, that induce alterations of consciousness similar to those documented for ritual ingestion of traditional entheogens.
— Ruck et al, 1979, Journal of Psychedelic Drugs