Magus, plural Magi, names the priestly office of Zoroastrianism, an office whose Greek speaking reputation for ritual knowledge, astrology and fire tending gave the English language the word magic. The Magi began as a priestly clan of the Medes, the people of ancient Media in what is now northwestern Iran, serving religious and funerary rites that predate Zoroaster's teaching; the Bisitun inscription of Darius the Great even records a Magian revolt against Achaemenid rule in 522 BCE, suggesting a priesthood with independent standing rather than one simply created by the Zoroastrian faith it later came to serve.
As Zoroastrianism spread and consolidated across the Persian world, the Magi became identified as its priesthood proper, responsible above all for tending the sacred fires that remain the religion's central ritual focus, along with funerary rites, purification and the recitation of the Avesta's liturgical texts. Modern Zoroastrian priesthood preserves a graded structure inherited from this history: the ervad is a priest in minor orders qualified for basic ritual duties, the mobed is a fully ordained priest able to perform the complete range of ceremonies, and the dastur is the senior rank associated with a fire temple housing the religion's highest grade of consecrated fire.
Mary Boyce's foundational study of Zoroastrian belief and practice traces this priesthood's continuity across nearly three thousand years of documented history, from its pre-Zoroastrian Median origins through its role as guardian of the empire's official religion under the Achaemenid, Parthian and Sasanian dynasties, to its much reduced but unbroken survival into the present day.