The heretical Christian belief that God/Jesus has two natures:
(1) a human nature that is embodied in Jesus Christ who was born and died on the cross and
(2) an immortal and eternal divine nature.
1. The doctrine was proposed by Nestorius (Patriarch of Constantinople from 428–431 CE), and it was opposed by Cyril of Alexandria. It was deemed heretical at the First Council of Ephesus (431) and at the Council of Chalcedon (451). This led to a schism in which churches supporting Nestorius — of which most were in the east — broke with the rest.
2. Nestorianism is a form of dyophysitism, and can be seen as the antithesis to monophysitism, which emerged in reaction to Nestorianism. Where Nestorianism holds that Christ had two loosely-united natures, divine and human, monophysitism holds that he had but a single nature, his human nature being absorbed into his divinity.
3. The Nestorian heresy maintained that "there were two entirely separate persons in the incarnate Christ, one human, the other divine, in opposition to the Orthodox belief that Christ was a single person, at once human and divine. " (William Dalrymple, From the Holy Mountain, 140)