An autarch was an absolute and dictatorial ruler.
After Benjamin Sisko was told by Ambassador Krajensky that the Tzenkethi Autarch had apparently been overthrown by a coup d'etat on Tzenketh in 2371, Sisko was pleased, saying that "I doubt anyone is going to miss the Autarch". (DS9: "The Adversary")
The Ilari in the Delta Quadrant were ruled by an autarch. The term autarch on Ilari appeared to be equivalent to the title emperor or empress on Earth, since the Autarch of Ilari resided in the Imperial Palace. The title also seems to be hereditary, passing to the firstborn child of the deceased monarch. (VOY: "Warlord")