The Eastern Orthodox Church, officially the Orthodox Catholic Church, also referred to as the Orthodox Church, Eastern Orthodoxy, and Orthodoxy, is the second largest Christian Church in the world, with an estimated 225–300 million adherents. It tends to be the Church identified most often with the single term "Orthodox", as it is the largest among the Christian groups that call themselves by this name.
The Eastern Orthodox Church is one of the oldest religious institutions in the world, teaching that it is the continuation of the One, Holy, Catholic and Apostolic Church established by Jesus Christ in his Great Commission to the apostles, and practicing what it understands to be the original faith passed down from the Apostles (Holy Tradition).
United in communion with the Latin Church for the first half of its history and with the Oriental churches for the first quarter of its history, 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 has resided in Constantinople since that city's founding in AD 324, centuries before the East–West Schism around 1054. Nevertheless, Orthodoxy has no Papacy or bishopric of similar authority. The commonly-used but unofficial designation of "eastern" derives from its geographical placement in relation to the "western" churches, which became (Roman) Catholic, and the historical function of Constantinople as the capital city of the eastern part of the Roman Empire.