The Chalcedonian Definition (also called the Chalcedonian Creed) was adopted at the Council of Chalcedon in A.D. 451. Chalcedon was an early centre of Christianity located in Asia Minor (modern Turkey). The Council was the fourth of the Ecumenical Councils that are accepted by Chalcedonian churches which include the Eastern Orthodox, Catholic, and most Protestant churches. It was the first Council not to be recognised by any Oriental Orthodox church; these churches may be classified as non-Chalcedonian.
The Definition defines that Christ is 'acknowledged in two natures', which 'come together into one person and one hypostasis'. The formal definition of 'two natures' in Christ was understood by the critics of the council at the time, and is understood by many historians and theologians today, to side with western and Antiochene Christology and to diverge from the teaching of Cyril of Alexandria, who always stressed that Christ is 'one'. However, a modern analysis of the sources of the creed (by A. de Halleux, in Revue Theologique de Louvain 7, 1976) and a reading of the acts, or proceedings, of the council (recently translated into English) show that the bishops considered Cyril the great authority and that even the language of 'two natures' derives from him.
Chalcedonian Definition Wikipedia
The Chalcedonian Definition was written amid controversy between the western and eastern churches over the meaning of the Incarnation (see Christology), the ecclesiastical influence of the emperor, and the supremacy of the Bishop of Rome. The western churches readily accepted the creed, but some eastern churches did not.
It became standard orthodox doctrine. However the Coptic Church of Alexandria dissented, holding to Cyril of Alexandria's preferred formula for the oneness of Christ’s nature in the incarnation of God the Word as "out of two natures". Cyril's language is not consistent and he may have countenanced the view that it is possible to contemplate in theory two natures after the incarnation, but the Church of Alexandria felt that the Definition should have stated that Christ be acknowledged "out of two natures" rather than "in two natures".
This miaphysite position, historically characterised by Chalcedonian followers as "monophysitism" though this is denied by the dissenters, formed the basis for the distinction from other churches of the Coptic Church of Egypt and Ethiopia and the "Jacobite" churches of Syria and Armenia (see Oriental Orthodoxy).
The key section runs:
Following, then, the holy Fathers, we all unanimously teach that our Lord Jesus Christ is to us One and the same Son, the Self-same Perfect in Godhead, the Self-same Perfect in Manhood; truly God and truly Man; the Self-same of a rational soul and body; co-essential with the Father according to the Godhead, the Self-same co-essential with us according to the Manhood; like us in all things, sin apart; before the ages begotten of the Father as to the Godhead, but in the last days, the Self-same, for us and for our salvation (born) of Mary the Virgin Theotokos as to the Manhood; One and the Same Christ, Son, Lord, Only-begotten; acknowledged in Two Natures unconfusedly, unchangeably, indivisibly, inseparably; the difference of the Natures being in no way removed because of the Union, but rather the properties of each Nature being preserved, and (both) concurring into One Person and One Hypostasis; not as though He were parted or divided into Two Persons, but One and the Self-same Son and Only-begotten God, Word, Lord, Jesus Christ; even as from the beginning the prophets have taught concerning Him, and as the Lord Jesus Christ Himself hath taught us, and as the Symbol of the Fathers hath handed down to us.
The full text of the definition which reaffirms the decisions of the Council of Ephesus, the pre-eminence of the Creed of Nicea (325) and the further definitions of the Council of Constantinople (381) can be found here It also affirms the authority of two of Cyril of Alexandria's letters and the Tome of Leo written against Eutyches and sent to Archbishop Flavian of Constantinople in 449.