Viceroy is a royal title given to a person who rules over a province or region as the agent of a sovereign (a "vice-royal").
In 2373, the Viceroy of Denar of the planet Ilari was prepared to provide Tieran political (and potentially military) support in exchange for more territory. (VOY: "Warlord")
In 2379, a Reman acted in the capacity of a viceroy to Praetor Shinzon during Shinzon's short-lived coup d'état against the Romulan government. (Star Trek Nemesis)