a sexual ritual that plays out a marriage between a god and a goddess, especially when enacted in a symbolic ritual where human participants represent the deities.
The notion of hieros gamos does not presuppose actual performance in ritual, but is also used in purely symbolic or mythological context, notably in alchemy and hence in Jungian psychology.
Langdon had read descriptions of this ceremony and understood its mystic roots. "It's called Hieros Gamos," he said softly. "It dates back more than two thousand years. Egyptian priests and priestesses performed it regularly to celebrate the reproductive power of the female."
Dan Brown. The Da Vinci Code, p308 (2007)
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The union of two things, the sun with the moon, the soul with God, the seer with the seen, etc., is also taught by the image of the connection of man and woman. That is the mystic marriage (Hieros gamos), a universally widespread symbol of quite supreme importance. In alchemy the last process, i.e., according to the viewpoint of representation, the tincturing or the unification, is quite frequently represented in the guise of a marriage—sometimes of a king and a queen. We cannot interchange this final process with the initial one of introversion, which (as a seeking for the uterus for the purpose of a rebirth) is likewise readily conceived of as a sexual union. If the symbol of coitus was conceivable there, so here, too, the same symbol is appropriate for the representation of the definite union with the object longed for.
Herbert Silberer. Hidden Symbolism of Alchemy and the Occult Arts (Jelliffe translation) (1971)