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 New International Dictionary of the English Language

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.