1) a tetrarch
1a) a governor of the fourth part of a region. Thus Strabo states that Galactia was formerly divided into three parts, each one of which was distributed into four smaller subdivisions each of which was governed by a tetrarch. Strabo relates that Thessaly, before the time of Philip of Macedon, had been divided into four tetrarchies, each having its own tetrarch.
1b) the governor of a third part or half a country, or even a ruler of an entire country or district provided it were of comparatively narrow limits; a petty prince. Thus Antony made Herod (afterwards king) and Phasael, sons of Antipater, tetrarchs of Palestine. After the death of Herod the Great, his sons, Achelaus styled an ethnarch but Antipas and Philip with the title of tetrarchs, divided and governed the kingdom left by their father.
Part of Speech: noun masculine
From G5064 and G757; the ruler of a fourth part of a country ("tetrarch"): - tetrarch.
These two sites give similar information, with the definition from several dictionaries and statistics on the use of the word.
Perseus Digital Library
University of Chicago's Logion lexicon
Thayer's dictionary plus other information.
From this site's dictionary (in Italian)
Click on the first column to search for that word as a form of the root τετραάρχης; click on the second column to search for that grammatical form of the root τετραάρχης; click on the third column to search for that word and grammatical form; click on the total to search for the root τετραάρχης.