Entheogens are psychoactive substances used in spiritual and religious contexts to induce altered states of consciousness. Hallucinogens such as the psilocybin found in so-called "magic" mushrooms have been used in sacred contexts since ancient times. Derived from a term meaning "generating the divine from within", entheogens are used supposedly to improve transcendence, healing, divination and mystical insight.

Entheogens have been used in religious rituals in the belief they aid personal spiritual development. Anthropological study has established that entheogens are used for religious, magical, shamanic, or spiritual purposes in many parts of the world. Civilizations such as the Maya and Aztecs used psilocybin mushrooms, peyote, and morning glory seeds in ceremonies meant to connect with deities and perform healing. They have traditionally been used to supplement diverse practices, such as transcendence, including healing, divination, meditation, yoga, sensory deprivation, asceticism, prayer, trance, rituals, chanting, imitation of sounds, hymns like peyote songs, drumming, and ecstatic dance.

In ancient Eurasian and Mediterranean societies, scholars hypothesized the sacramental use of entheogens in mystery religions, such as the Eleusinian Mysteries of ancient Greece. According to The Road to Eleusis, psychoactive kykeon brews may have been central to these rites, aimed at inducing visionary states and mystical insight. These interpretations emphasize entheogens as central to religious practices in antiquity.

In recent decades, entheogens have experienced a resurgence in academic and clinical research, particularly in psychiatry and psychotherapy. Preliminary clinical research indicates that substances such as psilocybin and MDMA may be useful in treating mental health conditions like depression, post-traumatic stress disorder, and anxiety, especially in end-of-life care. These developments reflect a broader reevaluation of entheogens not only as sacred tools but also as potentially transformative therapeutic agents.

The psychedelic experience is often compared to non-ordinary forms of consciousness such as those experienced in meditation, near-death experiences, and mystical experiences. Ego dissolution is often described as a key feature of the psychedelic state often resulting in perceived personal insight spiritual awakening, or a reorientation of values. Though evidence is often fragmentary, ongoing research in fields like archaeology, anthropology, psychology, and religious studies continues to shed light on the widespread historical and contemporary role of entheogens in human culture.

The term entheogen was coined in the late 20th century as a more neutral and respectful alternative to terms like "hallucinogen" or "psychedelic." The word is derived from the Greek words ἐν (en, "within"), θεός (theos, "god"), and γεννάω (gennao, "to generate"), meaning "generating the divine within." This term emphasizes the spiritual and religious contexts in which these substances have traditionally been used, distinguishing them from purely recreational or pharmacological classifications. The Greeks used it as praise for poets and other artists. Genesthai means "to come into being". Together, the term entheogen refers to a substance that "generates the divine within," typically producing feelings of inspiration, religious ecstasy, or spiritual insight.

The term hallucinogen was deemed 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 about psychosis and also because it had become irreversibly associated with various connotations of the 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 was formally defined:

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.