Easton's Bible Dictionary: M. G. Easton, M.A., D.D.
Strictly the ruler over the fourth part of a province; but the word denotes a ruler of a province generally (Mt. 14:1; Lk. 3:1, 19, 9:7; Acts 13:1). Herod and Phasael, the sons of Antipater, were the first tetrarchs in Palestine. Herod the tetrarch had the title of king (Mt. 14:9).