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Orthodox Church

noun

the Christian church comprising the local and national Eastern churches that are in communion with the ecumenical patriarch of Constantinople; Byzantine Church.
(originally) the Christian church of those countries formerly comprising the Eastern Roman Empire and of countries evangelized from it; Greek Orthodox Church.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for orthodox church

Orthodox Church

noun

Also called: Byzantine Church, Eastern Orthodox Church, Greek Orthodox Church the collective body of those Eastern Churches that were separated from the western Church in the 11th century and are in communion with the Greek patriarch of Constantinople
any of these Churches
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012