Easton's Bible Dictionary Strictly the ruler over the fourth part of a province; but the word denotes a ruler of a province generally (Matthew 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 (Matthew 14:9). Noah Webster's Dictionary 1. (n.) A Roman governor of the fourth part of a province; hence, any subordinate or dependent prince; also, a petty king or sovereign. 2. (a.) Four. Int. Standard Bible Encyclopedia TETRARCH te'-trark, tet'-rark tetrarches): As the name indicates it signifies a prince, who governs one-fourth of a domain or kingdom. The Greeks first used the word. Thus Philip of Macedon divided Thessaly into four "tetrarchies." Later on the Romans adopted the term and applied it to any ruler of a small principality. It is not synonymous with "ethnarch" at least the Romans made a distinction between Herod "tetrarch" of Galilee, Philip "tetrarch" of Trachonitis, Lysanias "tetrarch" of Abilene, and Archelaius "ethnarch" of Judea (BJ, II, vi, 3; Ant, XVII, xi, 4). The title was often conferred on Herodian princes by the Romans, and sometimes it was used courteously as a synonym for king (Matthew 14:9 Mark 6:14). In the same way a "tetrarchy" was sometimes called a kingdom. Henry E. Dosker

Multi-Version Concordance Tetrarch (5 Occurrences) Matthew 14:1 At that time, Herod the tetrarch heard the report concerning Jesus, (WEB KJV WEY ASV DBY WBS YLT NAS RSV NIV) Luke 3:1 Now in the fifteenth year of the reign of Tiberius Caesar, Pontius Pilate being governor of Judea, and Herod being tetrarch of Galilee, and his brother Philip tetrarch of the region of Ituraea and Trachonitis, and Lysanias tetrarch of Abilene, (WEB KJV WEY ASV DBY WBS YLT NAS RSV NIV) Luke 3:19 but Herod the tetrarch, being reproved by him for Herodias, his brother's wife, and for all the evil things which Herod had done, (WEB KJV WEY ASV DBY WBS YLT NAS RSV NIV) Luke 9:7 Now Herod the tetrarch heard of all that was done by him; and he was very perplexed, because it was said by some that John had risen from the dead, (WEB KJV WEY ASV DBY WBS YLT NAS RSV NIV) Acts 13:1 Now in the assembly that was at Antioch there were some prophets and teachers: Barnabas, Simeon who was called Niger, Lucius of Cyrene, Manaen the foster brother of Herod the tetrarch, and Saul. (WEB KJV WEY ASV DBY WBS YLT NAS RSV NIV)