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, 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 (Matt. 14:9).
Tetrarch
1 verse
Matthew
Matthew 14:1
At that time Herod the tetrarch heard of the fame of Jesus,
From Easton's Bible Dictionary
Matthew George Easton, 1897