Hypostasis (plural: hypostases), from the Greek ὑπόστασις (hypóstasis), is the underlying, fundamental state or substance that supports all of reality. It is not the same as the concept of a substance. In Neoplatonism, the hypostasis of the soul, the intellect (nous) and "the one" was addressed by Plotinus. In Christian theology, the Holy Trinity consists of three hypostases: that of the Father, that of the Son, and that of the Holy Spirit.
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The Second Council of Ephesus was a Christological church synod in 449 convened by Emperor Theodosius II under the presidency of Pope Dioscorus I of Alexandria. It was intended to be an ecumenical council, and it is accepted by the miaphysite churches, who view it as a valid continuation of the First Council of Ephesus if not an ecumenical council in its own right. The Second Council of Ephesus was explicitly repudiated by the next council, the Council of Chalcedon of 451. The Council of Chalcedon is recognized as the fourth ecumenical council by Chalcedonian Christians, and the Second Council of Ephesus was named the Latrocinium ("Robber Synod") by Pope Leo I; the Chalcedonian churches, particularly the Roman Catholic and Eastern Orthodox communions, continue to accept this designation, while the Oriental Orthodox repudiate it.
Both this council and that at Chalcedon dealt primarily with Christology, the study of the nature of Christ. Both councils affirmed the doctrine of the hypostatic union and upheld the orthodox Christian doctrine that Jesus Christ is both fully God and fully man. The Second Council of Ephesus decreed the formula of Cyril of Alexandria, stating that Christ is one incarnate nature [mia physis] (a qualitative description of the union of divinity and humanity), fully human and fully God, united without separation, without confusion, without mixture and without alteration. The Council of Chalcedon decreed that in Christ two natures exist, "a divine nature [physis] and a human nature [physis], united in one person [hypostasis], with neither division nor confusion".