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).
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Tetrarch'". "Easton's Bible Dictionary".