This terminology is often chosen to contrast with recreational use of the same substances.
The term "entheogen" was coined as a replacement for the terms "hallucinogen" (associated with delirium and insanity) and "psychedelic" (associated with psychosis, and '60s drug culture).
An entheogen "creates the divine within," en εν- "in, within," theo θεος- "god, divine," -gen γενος "creates, generates". In practice, the process is more an invocation of the divine - an absorption or collection from without - rather than an evocation, or creation originating within the person. So, an entheogen is something that fills someone with divinity.
Entheogens are tools to aid healing and transcendence, including in meditation, psychonautics, art projects, and psychedelic therapy.
Entheogens have been used in a ritualized context for thousands of years; their religious significance is well established in anthropological and modern evidences.
Historically, entheogens were mostly derived from plant sources and have been used in a variety of traditional religious contexts. They may be compounded by a shaman or apothecary in a tea, admixture, or potion like Ayahuasca/Yage, Bhang, or an anointing oil.
With the advent of organic chemistry, there now exist many synthetic substances with similar psychoactive properties. Most entheogens do not produce drug dependency.
Most of the well-known modern examples of entheogens, such as peyote, psilocybe and other psychoactive mushrooms and ololiuhqui, are from the native cultures of the Americas. However, it has also been suggested that entheogens played an important role in ancient Indo-European culture, for example by inclusion in the ritual preparations of the Soma, the "pressed juice" that is the subject of Book 9 of the Rig Veda. Soma was ritually prepared and drunk by priests and initiates and elicited a paean in the Rig Veda that embodies the nature of an entheogen:
Splendid by Law! declaring Law, truth speaking, truthful in thy works, Enouncing faith, King Soma!... O Soma Pavāmana (mind clarifying), place me in that deathless, undecaying world wherein the light of heaven is set, and everlasting lustre shines.... Make me immortal in that realm where happiness and transports, where joy and felicities combine…
Other older entheogens include the Kykeon that preceded initiation into the Eleusinian Mysteries, and the poppy, Datura, the unidentified "lotus" eaten by the Lotus-Eaters in the Odyssey and Narkissos in the Ancient Near East and the Aegean. Also the Indo Europeans brought knowledge of the wild Amanita mushroom, which could not be cultivated, and so had to be found, which suited it to a nomadic lifestyle. It has been suggested that Amanita was divine food, the food of the gods, their ambrosia, and it mediated between the two realms.
The entheogen is believed to offer godlike powers in many traditional tales, including immortality. The failure of Gilgamesh in retrieving the plant of immortality from beneath the waters teaches that the blissful state cannot be taken by force or guile: when Gilgamesh lay on the bank, exhausted from his heroic effort, the serpent came and ate the plant.
Naturally occurring entheogens such as psilocybin, and DMT (in the preparation ayahuasca), were for the most part discovered and used by older cultures, as part of their spiritual and religious life, as plants and agents which were respected, or in some cases revered for generations. Such traditional use in rites may predate all modern religions.
Entheogens have played a pivotal role in the spiritual practices of most American cultures for millennia. Peyote cactus is a recent example, which in recent centuries replaced the earlier toxic entheogen, the mescal bean. Mexican cultures also used psilocybin mushrooms, morning glory seeds, and salvia divinorum (divine sage).
Other South American entheogens include: borrachero (Brugmansia spp); San Pedro (Trichocereus spp); and various tryptamine-bearing snuffs, for example Epená (Virola spp), Vilca and Yopo (Anadananthera spp).
The best-known entheogen-using culture of Africa is the Bwitists, who used a preparation of the root bark of Iboga. The ancient Egyptians used the blue lotus.
Cannabis, in the form of Bhang, has long been consumed in the Hindu culture of the Indian subcontinent, and is associated with Lord Shiva, and the festival of Holi.
Similarly, Cannabis is used as an entheogens by more contemporary religious movements, such as the Rastafari movement and the Church of the Universe.
Cannabis has also been associated with holy anointing oil mentioned in various sacred Hebrew texts, and there is evidence suggesting use of additional visionary plants such as henbane, as well.
adapted from http://en.wikipedia.org/wiki/Entheogen