A theological term used to express the revealed truth that the one divine person or hypostasis of the Word subsists in two natures, divine and human. As the culmination of controversies spanning more over four hundred years, the dogmatic declaration of this truth of faith at the Council of Chalcedon (c. 451) represents a victory of Catholic orthodoxy over those accounts of Christology which would deny Christ’s true divinity (e.g., Arianism), deny his true humanity (e.g., Apollinarianism, Monophysitism), or reduce Christ’s unity to a kind of moral, non-personal union (e.g., Nestorianism).  The fifth and, especially, sixth ecumenical councils (Constantinople II in 553 and Constantinople II in 680–681) developed the affirmation of Christ’s duality of natures in a single divine person or hypostasis by clarifying that He has two wills and two will-acts, human and divine.  The anti-iconoclastic teachings and canons of the seventh ecumenical council (Nicaea II in 787) were a further affirmation of the implications of the hypostatic union of the human and divine in Christ.