The Eastern Orthodox Church, also known as the Orthodox Church, or officially as the Orthodox Catholic Church, is the second largest Christian church and one of the oldest current religious institutions in the world. The Eastern Orthodox Church teaches that it is the One, Holy, Catholic and Apostolic Church established by Jesus Christ in his Great Commission to the apostles. It practises what it understands to be the original Christian faith and maintains the sacred tradition passed down from the apostles.
The Eastern Orthodox Church is a communion of autocephalous churches, each typically governed by a Holy Synod. It teaches that all bishops are equal by virtue of their ordination, and has no central governing structure analogous to the Papacy in the Roman Catholic Church. The contemporary Orthodox Church had shared communion with the Roman Catholic Church until the East–West Schism starting around AD 1054, which had been triggered by disputes over doctrine, especially the authority of the Pope. Prior to the Council of Chalcedon in AD 451, the Eastern Orthodox had also shared communion with the Oriental Orthodox churches, separating primarily over differences in Christology.
Eastern Orthodoxy spread throughout the Roman and later Byzantine Empires and beyond, playing a prominent role in European, Near Eastern, Slavic, and some African cultures. Its most prominent episcopal see is Constantinople. The majority of Eastern Orthodox Christians live in Greece, Eastern Europe, the Caucasus and Russia, with less numerous communities in the former Byzantine regions of the Middle East and around the Eastern Mediterranean. There are also many in other parts of the world, formed through immigration, conversion and missionary activity.