Originally a patriarch was a man who exercised autocratic authority as a pater familias over an extended family. The system of such rule of families by senior males is called patriarchy. This is a Greek word, a composition of (pater) meaning "father" and (archon) meaning "leader", "chief", "ruler", "king", etc.

Abraham, Isaac, and Jacob are referred to as the three patriarchs of the people of Israel, and the period in which they lived is called the Patriarchal Age. It originally acquired its religious meaning in the Septuagint version of the Bible.

The word has mainly taken on specific ecclesiastical meanings. In particular, the highest-ranking bishops in Eastern Orthodoxy, Oriental Orthodoxy, the Roman Catholic Church (above Major Archbishop and primate), and the Assyrian Church of the East are called patriarchs. The office and ecclesiastical conscription (comprising one or more provinces, though outside his own (arch)diocese he is often without enforceable jurisdiction) of such a patriarch is called a patriarchate. Historically, a Patriarch may often be the logical choice to act as Ethnarch, representing the community that is identified with his religious confession within a state or empire of a different creed (as Christians within the Ottoman Empire).