The Patriarch is the highest-ranking bishop in a self-governing and historically significant Christian center. The most famous are the ancient patriarchates of Constantinople, Alexandria, Antioch, and Jerusalem, which are Eastern Orthodox jurisdictions.
The head of the Roman Catholic Church, the Pope, is styled the "Patriarch of the West" while the leader of Eastern Orthodoxy, the Patriarch of Constantinople, is styled the "Ecumenical Patriarch." In more recent years, newer bishoprics have had the title of "patriarchate" given to them. Examples are the Orthodox churches in Russia (i.e. Moscow), Serbia, Romania, Bulgaria, and Georgia, plus the Roman Catholic Church's own patriarchate of Jerusalem.
These men had many sons and ruled over large families. Analogously, one might today use the term "patriarch" to refer to the male head of a large and influential family, such as George H.W. Bush or Joseph Kennedy.