a governor: an official a king or empire appoints to administer a province or command a region on the ruler's behalf
This is a borrowed administrative title, not a native Hebrew word, and the Bible uses it for the appointed officials of the great empires that ruled over Israel and its neighbors: Assyria, Babylon, and above all Persia. Whoever holds the office answers upward to a greater power. A governor is never an independent king in his own right; he is a delegate, running a territory for the crown that placed him there.
The title stretches across a range of ranks, which is why English translations reach for more than one word. At the top it names the head of a whole province, the person responsible for its taxes, order, and loyalty to the empire. Zerubbabel and later Nehemiah held exactly this post over Judah after the exile, and the same word covers the governors of the wider region "Beyond the River" and the provincial governors scattered through the Persian empire in Esther. Lower down, the same word can shrink to a lesser official or even a mid-level army officer, so that a boastful Assyrian commander can sneer at "a single captain" among his master's least servants.
The two uses are the same office seen at different levels, held together by one idea: authority handed down from a superior power and exercised over a place and its people on that power's behalf.