His Holiness, the Catholicos of Armenia and of All Armenians (plural Catholicoi, due to its Greek origin) is the head bishop of Armenia's dominant church, the Armenian Apostolic Church. That church is one of the Oriental Orthodox churches that separated from the rest of the Christian church in 451 as a result of the decisions of the Council of Chalcedon.
He is also sometimes called the "Catholicos of Etchmiadzin" after the city that is his see.
See also