An autarch is an absolute, dictatorial ruler, sometimes placed into power through his or her own charisma or self-ambition.
In 2371, Federation ambassador Krajensky informed Deep Space 9 Captain Benjamin Sisko the Tzenkethi Autarch had been overthrown in a coup d'etat, Sisko responded, "I doubt anyone is going to miss the Autarch". (DS9: "The Adversary")
Cantabrian commanding officer Noah Wrightson and counselor Daniel Radke, as escaped captives aboard the Myhr'an super battle cruiser in late 2372, discovered the Myhr'an commander was the Myhr'an Autarch while battling him. In return for his co-operation with the M'Tar, the Myhr'an commander had allowed millions of his people to be slaughtered and replaced by Myhr'an clones; he died when his ship was severely crippled and subsequently exploded in battle with the Cantabrian. (Star Trek: The Cantabrian Expeditions: "Deceptions")