in Greco-Roman antiquity, the ruler of a principality; originally the ruler of one-quarter of a region or province. The term was first used to denote the governor of any of the four tetrarchies into which Philip II of Macedon divided Thessaly in 342 BC—namely, Thessaliotis, Hestiaeotis, Pelasgiotis, and Phthiotis. (These may, however, have constituted a revival of a division of earlier origin.) Later, the term tetrarchy was applied to the four divisions of Galatia (in Anatolia) before its conquest by the Romans (169 BC).

Even later, “tetrarch” became familiar as the title of certain Hellenized rulers of petty dynasties in Syria and Palestine, whom the Romans allowed a measure of independent sovereignty. In this usage it lost its original precise sense and meant only the ruler of a divided kingdom or of a district too minor to justify a higher title. After the death of Herod the Great (4 BC), his realm was shared among his three sons: the chief part, including Judaea, Samaria, and Idumaea, fell to Archelaus, with the title of ethnarch; Philip received the northeast of the realm and was called tetrarch; and Galilee was given to Herod Antipas, who also was called tetrarch. These three sovereignties were reunited under Herod Agrippa from AD 41 to 44.