council · 451 · Chalcedon
The largest council of antiquity — about 520 bishops — met at Chalcedon in 451 under Marcian and Pulcheria. It received the Tome of Leo, defined one and the same Christ in two natures without confusion or separation, deposed Dioscorus of Alexandria, and in canon 28 extended Constantinople's privileges. Egypt and much of Syria refused the Definition: the miaphysite churches walk apart from this day.
The Council of Chalcedon was the fourth ecumenical council of the Christian Church. It was convoked by the Roman emperor Marcian. The council convened in the city of Chalcedon, Bithynia from 8 October to 1 November 451. The council was attended by over 520 bishops or their representatives, making it the largest and best-documented of the first seven ecumenical councils. The principal purpose of the council was to re-assert the teachings of the ecumenical Council of Ephesus against the teachings of Eutyches and Nestorius. Such doctrines viewed Christ's divine and human natures as separate and distinct (Nestorianism), or viewed Christ as solely divine (monophysitism). The Council of Chalcedon issued the Chalcedonian Definition, stating that Jesus is "perfect both in deity and in humanness; this selfsame one is also actually God and actually man." The Council's judgments and definitions regarding the divine marked a significant turning point in the Christological debates.
After Eutyches and the Robber Council: how are Godhead and manhood united in Christ?
The Definition: one and the same Christ in two natures, without confusion, change, division or separation; Dioscorus deposed; canon 28 for New Rome; Jerusalem a patriarchate.
Wikipedia · Catholic Encyclopedia — Council of Chalcedon · OrthodoxWiki — Fourth Ecumenical Council · The Seven Ecumenical Councils — Chalcedon (NPNF II/14)