Is strictly the ruler of the fourth part of a state or province; but in the New Testament it is a general title applied to those who governed any part of a kingdom or province, with an authority subject only to that of the Roman emperor. Thus Herod the Great and his brother were at one time, in early life, constituted tetrarchs of Judea by Antony. At the death of Herod the Great, he left half his kingdom to Archelaus, with the title of ethnarch; while the other half was divided between two of his other sons. Herod Antipas and Philip, with the title of tetrarchs. See HEROD 1,2.
In the same manner Lysanias is also said to have been tetrarch of Abilene, Luke 3:1 . It is Herod Antipsas who is called the tetrarch in Matthew 14:1 Luke 3:19 9:7 Acts 13:1 . As the authority of the tetrarch was similar to that of the king, so the general term king is also applied to Herod, Matthew 14:9 Mark 6:14 .