A Roman military rank, similar to the previous Greek usage

Tetrarch is a Greek term meaning "ruler of a quarter" for a holder of Imperial office under a Tetrarchy. It was applied earlier to rulers of minor principalities owing allegiance to Rome. It can also refer, more loosely, to

A provincial governor in a country composed of several smaller parts.

A subordinate or dependent prince, or petty sovereign. For example, the Tetrarchs of Roman Palestine, such as Herod Antipas, were tributary kings (Matt. 14:9).

A Macedonian officer in charge of a unit associated with the number four (tetra), such as a quarter of a larger unit (such as a phalanx, or in one case a 'company' of 64 footmen), or comprising four smaller units (in one case 4 lochoi, each under a lochagos).

A British light tank of World War II, see Tetrarch tank

A book (see Tetrarch (Well of Echoes)) by Ian Irvine, part of the Well of Echoes series