Eastern Christianity comprises Christian traditions and church families that originally developed during classical and late antiquity in Eastern Europe, Southeastern Europe, Asia Minor, the Caucasus, Northeast Africa, the Fertile Crescent and the Malabar coast of South Asia, and ephemerally parts of Persia, Central Asia, the Near East and the Far East. The term does not describe a single communion or religious denomination.

Major Eastern Christian bodies include the Eastern Orthodox Church and the Oriental Orthodox Churches, along with those groups descended from the historic Church of the East, as well as the Eastern Catholic Churches (which have either re-established or always retained communion with Rome and maintain Eastern liturgies), and the Eastern Protestant churches (which are Protestant in theology but Eastern in cultural practice). The various Eastern churches do not normally refer to themselves as "Eastern", with the exception of the Assyrian Church of the East and the Ancient Church of the East.

The Eastern Orthodox are the largest body within Eastern Christianity with a worldwide population of 220 million, followed by the Oriental Orthodox at 60 million. The Eastern Catholic Churches consist of about 16-18 million and are a small minority within the Catholic Church. Eastern Protestant Christian churches do not form a single communion; churches like the Ukrainian Lutheran Church and Mar Thoma Syrian Church have under a million members. The Assyrian Church of the East and the Ancient Church of the East, descendant churches of the Church of the East, have a combined membership of approximately 400K.

Historically, after the loss of the Levant in the 7th century to the Islamic Sunni Caliphate, the term Eastern Church was used for the Greek Church centred in Byzantium, in contrast with the (Western) Latin Church, centered on Rome, which uses the Latin liturgical rites. The terms "Eastern" and "Western" in this regard originated with geographical divisions in Christianity mirroring the cultural divide between the Hellenistic East and the Latin West, and the political divide of 395 AD between the Western and Eastern Roman empires. Since the Protestant Reformation of the 16th century, the term "Eastern Christianity" may be used in contrast with "Western Christianity", which contains not only the Latin Church but also Protestantism and Independent Catholicism. Some Eastern churches have more in common historically and theologically with Western Christianity than with one another.

Because the largest church in the East is the body currently known as the Eastern Orthodox Church, the term "Orthodox" is often used in a similar fashion to "Eastern", to refer to specific historical Christian communions. However, strictly speaking, most Christian denominations, whether Eastern or Western, regard themselves as "orthodox" (meaning "following correct beliefs") as well as "catholic" (meaning "universal"), and as sharing in the Four Marks of the Church listed in the Nicene-Constantinopolitan Creed (325 AD): "One, Holy, Catholic and Apostolic" (Greek: μία, ἁγία, καθολικὴ καὶ ἀποστολικὴ ἐκκλησία).

Eastern churches (excepting the non-liturgical dissenting bodies) utilise several liturgical rites: the Alexandrian Rite, the Armenian Rite, the Byzantine Rite, the East Syriac Rite (also known as Persian or Chaldean Rite), and the West Syriac Rite (also called the Antiochian Rite).