Church of the East

From Classical Syriac ܥܕܬܐ ܕܡܕܢܚܐ (ʿĒḏtā d-Maḏenḥā), so called as it predominated to the east of the border of the Christian Roman Empire. Used as a proper noun in English from c. 1890s.

the Church of the East (Christianity)

1. (historical) The church originally made up of the Christians of the Sasanid Empire, later exonymically known as the Nestorian church and characterised by its use of the East Syriac liturgical rite.
2. Any of the modern successors of that church following its schisms since the 16th century, in particular the Ancient Church of the East, Assyrian Church of the East, and Chaldean Catholic Church, or all of these considered together.