Lysanias Of Abilene
A "tetrarch" was a title given by the imperial Romans (see Ancient Empires - Rome) for one of their governors who was given to rule a fourth of a Roman "province" (a term that originated from a combination of two Latin words, pro, in that case meaning to, and vinco, meaning conquer). The Roman "tetrarch" originated from the Greek words tetra, meaning four, and arche, meaning rule, which to the Greeks (see Ancient Empires - Greece) was used to denote a commander of a fourth section of a phalanx - a battle formation of infantry consisting of swordsmen and spearmen that "spearheaded" a focused attack upon a more dispersed enemy force.
Perhaps the most famous tetrarch was Herod (not to be confused with "Herod the Great" who tried to have the infant Christ killed - see the Fact finder question below) who was tetrarch of Galilee at the time that The Prophet Of Galilee began to preach.
"14:1 At that time Herod the tetrarch heard of the fame of Jesus, 14:2 And said unto his servants, This is John the Baptist; he is risen from the dead; and therefore mighty works do show forth themselves in him. 14:3 For Herod had laid hold on John, and bound him, and put him in prison for Herodias' sake, his brother Philip's wife [Herodias had married two of her uncles - see Herodias]. 14:4 For John said unto him, It is not lawful for thee to have her." (Matthew 14:1-4 KJV)
"Lysanias the tetrarch of Abilene"
There were of course other tetrarchs at that time, including Lysanias who was "tetrarch of Abilene." Abilene was the name given to an area east of the Anti-Lebanon mountain range, named after its town, Abila.
"3:1 Now in the fifteenth year of the reign of Tiberius Caesar, Pontius Pilate being governor of Judaea, and Herod being tetrarch of Galilee, and his brother Philip tetrarch of Ituraea and of the region of Trachonitis, and Lysanias the tetrarch of Abilene, 3:2 Annas and Caiaphas being the high priests, the word of God came unto John the son of Zacharias in the wilderness [see The Prophets: John The Baptist].
John the Baptist was a Levite (see Rabbi John) who was feared and hated by the religious authorities of the land of Israel - men who got their "authority" from the tetrarchs.
"3:7 Then said he to the multitude that came forth to be baptized of him, O generation of vipers, who hath warned you to flee from the wrath to come? 3:8 Bring forth therefore fruits worthy of repentance, and begin not to say within yourselves, We have Abraham to our father: for I say unto you, That God is able of these stones to raise up children unto Abraham. 3:9 And now also the ax is laid unto the root of the trees: every tree therefore which bringeth not forth good fruit is hewn down, and cast into the fire.
It was nevertheless the tetrarchs that had John martyred (which literally means "witness" - see Martyrs).
"3:19 But Herod the tetrarch, being reproved by him for Herodias his brother Philip's wife, and for all the evils which Herod had done, 3:20 Added yet this above all, that he shut up John in prison [see How Long Was John's Ministry?]" (Luke 3:19-20 KJV)
Fact Finder: How many "Herods" are there in the New Testament record?