A patriarchate is the office or jurisdiction of a patriarch; a patriarch, as that term is used here, is either one of the highest-ranking bishops in Eastern Orthodoxy, of whom there were originally four, but now nine, or one of the ten highest-ranking bishops of Catholicism -- the pope, the seven "patriarchs of the east," and the bishops of Lisbon and Venice. The original four Catholic patriarchs of the east, and the original four Eastern Orthodox patriarchs, sit in Constantinople (now called Istanbul, but still called Constantinople in this particular ecclesiastical usage), Alexandria, Antioch, and Jerusalem. Eastern Orthodoxy recognizes not only its own patriarchs, but also the pope, in his role as Christian patriarch of Rome, as among the patriarchs whose presence at a council is a necessary condition for the council's ecumenicity and infallibility.
A patriarchate is treated in some legal juridictions as a corporate entity. For example, the Orthodox Patriarchate of Jerusalem filed a lawsuit in New York, decided in 1999, against Christie's Auction House, disputing the ownership of the Archimedes Palimpsest.