Definition of "haeresiarch"

Nestorǐus, a celebrated Haeresiarch, was appointed patriarch of Constantinople a.d. 428, but in consequence of his heresy was deposed at the council of Ephesus, 431. […] Nestorius carefully distinguished between the divine and human nature attributed to Christ, and refused to give to the Virgin Mary the title of Theotocus (Θεοτόκος) or "Mother of God."

1858, William Smith, “Nestorǐus”, in A Classical Dictionary of Biography, Mythology, and Geography, Based on the Larger Dictionaries, 4th revised edition, London: John Murray, […]; Walton and Maberly, […], page 476, column 1