The Eastern Orthodox Church, officially the Orthodox Catholic Church, and also called the Greek Orthodox Church or simply the Orthodox Church, is one of the three major doctrinal and jurisdictional groups of Christianity. As of 2012, it has approximately 300 million adherents and is the third largest religious community in the world after Roman Catholics and Sunni Muslims. The Eastern Orthodox Church operates as a communion of autocephalous churches, each governed by its bishops via local synods. The church has no central doctrinal or governmental authority analogous to the pope of the Catholic Church. Nevertheless, the Ecumenical Patriarch of Constantinople is recognised by them as primus inter pares ('first among equals'), a title held by the patriarch of Rome prior to 1054. As one of the oldest surviving religious institutions in the world, the Eastern Orthodox Church has played an especially prominent role in the history and culture of Eastern and Southeastern Europe. Since 2018, there has been an ongoing schism between Constantinople and Moscow, with the two not in full communion with each other.

Eastern Orthodox theology is based on the Scriptures and holy tradition, which incorporates the dogmatic decrees of the seven ecumenical councils, and the teaching of the Church Fathers. The church teaches that it is the one, holy, catholic and apostolic church established by Jesus Christ in his Great Commission, and that its bishops are the successors of Christ's apostles. It maintains that it practises the original Christian faith, as passed down by holy tradition. Its patriarchates, descending from the pentarchy, and other autocephalous and autonomous churches, reflect a variety of hierarchical organisation. It recognises seven major sacraments (which are called holy mysteries), of which the Eucharist is the principal one, celebrated liturgically in synaxis. The church teaches that through consecration invoked by a priest, the sacrificial bread and wine become the body and blood of Christ. The Virgin Mary is venerated in the Eastern Orthodox Church as the Theotokos, which means 'God-bearer', and she is honoured in devotions.

The churches of Constantinople, Alexandria, Jerusalem, and Antioch—except for some breaks of communion such as the Photian schism or the Acacian schism—shared communion with the Church of Rome until the East–West Schism in 1054. The 1054 schism was the culmination of mounting theological, political, and cultural disputes, particularly over the authority of the pope, between those churches. Before the Council of Ephesus in AD 431, the Church of the East also shared in this communion, as did the various Oriental Orthodox Churches before the Council of Chalcedon in AD 451, all separating primarily over differences in Christology.

The Eastern Orthodox Church is the primary religious confession in Russia, Ukraine, Romania, Greece, Belarus, Serbia, Bulgaria, Georgia, Moldova, North Macedonia, Cyprus, and Montenegro. Eastern Orthodox Christians are also one of the main religious groups in Estonia, Bosnia and Herzegovina, Kosovo and Latvia as well as a significant group in Syria, Lebanon, and other countries in the Middle East. Roughly half of Eastern Orthodox Christians live in the post Eastern Bloc countries, mostly in Russia. The communities in the former Byzantine regions of North Africa and the Eastern Mediterranean are among the oldest Orthodox communities from the Middle East, which are decreasing due to forced migration driven by increased religious persecution. Eastern Orthodox communities outside Western Asia, Asia Minor, Caucasia and Eastern Europe, including those in North America, Western Europe, and Australia, have been formed through diaspora, conversions, and missionary activity.

The Eastern Orthodox Church is defined as the Eastern Christians which recognise the seven ecumenical councils and usually are in communion with the Ecumenical Patriarchate, the Patriarchate of Alexandria, the Patriarchate of Antioch, and the Patriarchate of Jerusalem. The Eastern Orthodox churches "are defined positively by their adherence to the dogmatic definitions of the seven [ecumenical] councils, by the strong sense of not being a sect or a denomination but simply continuing the Christian church, and, despite their varied origins, by adherence to the Byzantine rite". Those churches are negatively defined by their rejection of papal immediate and universal supremacy.

The seven ecumenical councils recognised by the Eastern Orthodox churches are: Nicaea I, Constantinople I, Ephesus, Chalcedon, Constantinople II, Constantinople III, and Nicaea II. These churches consider the Quinisext Council as "shar[ing] the ecumenical authority of Constantinople III." "By an agreement that appears to be in place in the [Eastern] Orthodox world, possibly the council held in 879 to vindicate the Patriarch Photius will at some future date be recognized as the eighth [ecumenical] council" by the Eastern Orthodox Church.

Western Rite Orthodoxy exists both outside and inside Eastern Orthodoxy. Within Eastern Orthodoxy, it is practised by a vicariate of the Antiochian Orthodox church. Although there are roughly 3,000 members worldwide, spread across about 50 scattered parishes, Western Rite Orthodoxy remains a very small minority within the Eastern Orthodox Church.

In keeping with the church's teaching on universality and with the Nicene Creed, Eastern Orthodox authorities such as Raphael of Brooklyn have insisted that the full name of the church has always included the term "Catholic", as in "Holy Orthodox Catholic Apostolic Church".