Tet´rarch, a prince or sovereign who holds or governs a fourth part of a kingdom, without wearing the diadem or bearing the title of king. Such was the original import of the word, but it was afterwards applied to any petty king or sovereign, and became synonymous with ethnarch.
In the reign of Tiberius Caesar Herod's kingdom of Judea was divided into three parts, which were called tetrarchies, and the sovereigns tetrarchs. His sons were made the heirs to his kingdom. Archelaus became tetrarch of Judea, Samaria, and Idumea; Philip of Trachonitis and Ituraea; and Herod Antipas of Galilee and Peræa (). Herod Agrippa, the nephew of Herod Antipas, who afterwards obtained the title of king (), was in the reign of Caligula invested with royalty, and appointed tetrarch of Abilene; to which was afterwards added Galilee and Peræa, Judea and Samaria; until at length his dominion extended over the whole land of Palestine [HERODIAN FAMILY]. The title of tetrarch was frequently conferred upon the descendants of Herod the Great by the Roman emperors.