Adam Clarke on Matthew 14:1

Herod the tetrarch - This was Herod Antipas, the son of Herod the Great. See the notes on Mat 2:1, where an account is given of the Herod family. The word tetrarch properly signifies a person who rules over the fourth part of a country; but it is taken in a more general sense by the Jewish writers, meaning sometimes a governor simply, or a king; see Mat 14:9. The estates of Herod the Great were not, at his death, divided into four tetrarchies, but only into three: one was given by the Emperor Augustus to Archelaus; the second to Herod Antipas, the person in the text; and the third to Philip: all three, sons of Herod the Great.