Nontrinitarianism
Nontrinitarianism is a form of Christianity that rejects the mainstream Christian doctrine of the Trinity—the teaching that God is three distinct hypostases or persons who are coeternal, coequal, and indivisibly united in one being, or essence (from the Greek ousia). Certain religious groups that emerged during the Protestant Reformation have historically been known as antitrinitarian, but are not considered Protestant in popular discourse due to their nontrinitarian nature.
According to churches that consider the decisions of ecumenical councils final, Trinitarianism was definitively declared to be Christian doctrine at the 4th-century ecumenical councils, that of the First Council of Nicaea (325), which declared the full divinity of the Son, and the First Council of Constantinople (381), which declared the divinity of the Holy Spirit.