The Eastern Orthodox Church is a communion comprising the collective body of sixteen separate autocephalous hierarchical churches that recognize each other as "canonical" Orthodox Christian churches. The head of the communion is the Patriarch of Constantinople, who is also head of one of the sixteen churches. The sixteen organizations are in full communion with each other, so any priest of any of those churches may lawfully minister to any member of any of them, and no member of any is excluded from any form of worship in any of the others. Despite the fact that, like the Roman Catholic church, they are "closed communion" churches, i.e. with rare exceptions excluding non-members from receiving the Eucharist, nonetheless they admit each other's members to that sacrament. Friction among them is over matters of church politics rather than doctrine.
Like the Roman Catholic Church, the Eastern Orthodox Church claims to be the One Holy Catholic and Apostolic Church.
All the disagreements among persons of differing religious beliefs beget strange nomenclature, and accordingly the so-called Western Orthodox Church is a part of the Eastern Orthodox Church as that term is defined here.