Dionysus (/daɪ.əˈnaɪsəs/; Greek: Διόνυσος) is the god of the grape-harvest, winemaking and wine, of fertility, orchards and fruit, vegetation, insanity, ritual madness, religious ecstasy, festivity and theatre in ancient Greek religion and myth.

He is also known as Bacchus (/ˈbækəs/ or /ˈbɑːkəs/; Greek: Βάκχος, Bákkhos), the name adopted by the Romans; the frenzy he induces is bakkheia. Another name used by the Romans is Liber meaning "free", due to his association with wine and the Bacchanalia and other rites, and the freedom associated with it. His thyrsus, sometimes wound with ivy and dripping with honey, is both a beneficent wand and a weapon used to destroy those who oppose his cult and the freedoms he represents. As Eleutherios ("the liberator"), his wine, music and ecstatic dance free his followers from self-conscious fear and care, and subvert the oppressive restraints of the powerful. Those who partake of his mysteries are believed to become possessed and empowered by the god himself.

In his religion, identical with or closely related to Orphism, Dionysus was believed to have been born from the union of Zeus and Persephone, and to have himself represented a chthonic or underworld aspect of Zeus. Many believed that he had been born twice, having been killed and reborn as the son of Zeus and the mortal Semele. In the Eleusinian Mysteries he was identified with Iacchus, the son (or, alternately, husband) of Demeter.