Bible Dictionary · T
Tetrarch
Strictly the ruler over the fourth part of a province; but the word denotes a ruler of a province generally (Matt 14:1; Luke 3:1, Luke 3:19; Luke 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 (Matt 14:9).
From Easton’s Bible Dictionary (1897), by Matthew George Easton.