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The Chalcedonian Definition (also called the Chalcedonian Creed or the Definition of Chalcedon) is the declaration of Christ as fully God and fully human, having two natures “without confusion, without change, without division, without separation,” adopted at the Council of Chalcedon in AD 451. Chalcedon was an early centre of Christianity located in Asia Minor. The council was the fourth of the ecumenical councils that are accepted by Chalcedonian churches which include the Catholic and Orthodox churches.
It was the first council not to be recognised by any Oriental Orthodox church; for this reason these churches may be classified as Non-Chalcedonian.
General info from Wikipedia.org