A tetrarch was a type of Roman official who ruled part of a province. Tetrarchs were local leaders, but they were not powerful enough to be called kings. This title was used in Roman provinces like Thessaly, Galatia, and Syria.
The title "tetrarch" may have come from governors who ruled over a fourth part of a region or country, as in Syria after the death of Herod the Great. By New Testament times, the significance of that meaning had lessened. The title of tetrarch applied only to secondary princes by that time.
Three tetrarchs are mentioned in the Bible. Luke reports that Herod Antipas was the tetrarch of Galilee. He also identifies Philip as the tetrarch of Iturea and Traconitis, and Lysanius as the tetrarch of Abilene (Luke 3:1). The only tetrarch mentioned elsewhere in the Bible is Herod (Matthew 14:1; Luke 3:19; 9:7; Acts 13:1). Herod is also referred to as "king" by his Jewish subjects, showing his greater significance (Matthew 14:9; Mark 6:14).