||It has been suggested that this article be merged into Eastern Christianity. (Discuss) Proposed since December 2012.|
An Orthodox Church is "a Christian Church or federation of Churches originating in the Greek-speaking Church of the Byzantine Empire, not accepting the authority of the Pope of Rome, and using elaborate and archaic forms of service". Some Orthodox Churches originated also in non-Greek-speaking areas outside the Byzantine Empire such as Armenia and Ethiopia. Orthodox Churches belong mainly to two groups, Eastern Orthodoxy and Oriental Orthodoxy. The two groups are not in communion with each other.
Eastern Orthodoxy is the majority religion in Greece, Russia, Ukraine, Belarus, Bulgaria, Romania, Serbia, Georgia and a number of smaller countries. It is also a significant minority religion in the Middle East. Oriental Orthodoxy is the majority religion in Ethiopia and Armenia, a large minority religion in Egypt, and contains significant groups of believers in southern and south-western Asia.
The term orthodox is a combination of two Greek words orthos ("right", "true") and doxa ("belief", "opinion", or also "praise", "glory"), so that it can be interpreted as meaning "right belief" or "right praise".
Eastern Orthodox and Oriental Orthodox 
The Orthodox churches, together with what is now the Roman Catholic Church and the daughter-churches that originated from it after the Reformation, were united and single until the Council of Chalcedon in AD 451, after which the theological interpretation of the Christ as dyophysite and miaphysite divided them. The miaphysite body, corresponding largely to the patriarchates of Alexandria (Egypt, Ethiopia) and Antioch (Syria), and also Armenia, rejected the Chalcedon council and has come to be known as the Oriental Orthodox Church. The dyophysite body later split into what are now the Eastern Orthodox Church and the Roman Catholic Church.
The division between the Eastern Orthodox and Oriental Orthodox families of churches was brought about over the definition by the Council of Chalcedon that Jesus exists "in two natures", one human and one divine, and that "both natures concur in one person and in one reality [hypostasis]. They are not divided or cut into two persons, but are together the one and only and only-begotten Word, God, the Lord Jesus Christ." The Council of Chalcedon thus declared that Christ is one person in two natures "of one substance with the Father according to his divinity, of one substance with us according to his humanity ... in two natures without confusion, without change, without division, without separation."
The churches that accept the Council of Chalcedon's definition are known as Chalcedonian churches, and those that reject it as non-Chalcedonian or pre-Chalcedonian. The Chalcedonian churches in the East are known collectively as the Eastern Orthodox Church. (The Roman Catholic Church in the West is also a Chalcedonian Church, since it accepts that council's definition, which was largely based on a document of Pope Leo I.) Those that reject the Council of Chalcedon form what is known as Oriental Orthodoxy.
Dialogues aimed at achieving full communion between the Eastern and the Oriental Orthodox are in progress, with the hope of overcoming the schism that still divides them. These dialogues have led to a large measure of agreement, but not yet to full normal communion.
Apart from the use by the few parishes of Western Rite Orthodoxy of adapted or specially composed liturgies based on Latin liturgical rites, all the churches that form the Eastern Orthodox Church use the Byzantine Rite liturgy, celebrating it in different languages. The Oriental Orthodox Churches, on the contrary, use a great variety of liturgies.
Some churches of the Eastern Orthodox tradition are not in communion with the general body, usually because of disputes about the use of the Julian calendar, but in some cases because of political problems. There is one such case also in Oriental Orthodoxy, namely that of the Malabar Independent Syrian Church in India.
See also 
- "Definition of Orthodox Church". Oxford dictionaries online. Retrieved November 27, 2012.
- "The Oriental Orthodox churches, along with those of the Byzantine tradition or Eastern Orthodox, belong to the larger family of the Orthodox churches. The two groups are not in communion with each other" (Orthodox churches (Oriental)
- Religious Groups That Use 'Orthodox' in Their Names But Are Not Canonical Eastern Orthodox Churches
- David J. Goa, "Orthodox Church" in The Canadian Encyclopedia
- Definition concerning the two natures of Christ
- J. Martin Bailey. Who are the Christians in the Middle East? (Eerdmans 2003), p. 66
- James Leslie Houlden, Jesus in History, Thought and Culture: An Encyclopedia, volume 1, p. 666
- Ronald G. Roberson, The Eastern Christian Churches: a brief survey, p. 5
- Thomas E. Fitzgerald, The Ecumenical Movement: An Introductory History, p. 56]
- Pastoral Agreement between the Coptic Orthodox and Greek Orthodox Patriarchates of Alexandria (2001)
- Frequently Asked Questions About the Oriental Orthodox
- The Coptic Orthodox Church
- Growth in Agreement II, by Jeffrey Gros, Harding Meyer, William G. Rusch (Eerdmans Publishing, 2000 ISBN 978-2-8254-1329-6) chapter VIII: Eastern Orthodox-Oriental Orthodox Dialogue
- Texts of the Agreed Statements of the Joint Commission
- "The Orthodox all the world over must follow the Rite of Constantinople. In this unjustifiable centralization we have a defiance of the old principle, since Antioch, Jerusalem, Alexandria, Cyprus, in no way belong to the Byzantine Patriarchate" (Adrian Fortescue, Rites).
- "There are three Ethiopic liturgies all addressed to the Son. In Syria itself the Monophysite Second Liturgy of S. Peter ... (is) directed to the Son, as is part of the eucharistic prayer of the Syriac St James itself, which is followed in this by nearly all the sixty or seventy lesser Syriac liturgies" (The Shape of the Liturgy, by Gregory Dix, published by Continuum, 2005, page 180). The Coptic Church uses the Liturgy of St Cyril, the Liturgy of St Gregory the Theologian, and the Coptic Liturgy of St Basil. In Ethiopia, the most commonly used liturgy is the Ethiopian Orthodox Liturgy of the Apostles.