According to Wikipedia, an entheogen is "a psychoactive substance [. . .] that occasions enlightening spiritual or mystical experience." Specifically:
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.
The term 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)." Apparently it was intended as a more positive-connotation replacement for the terms "hallucinogen" and "psychedelic."