Castellan is a title, a type of governor. The term "castellan" describes a position in numerous races and cultures, although the word itself derives from old Earth usage, where a castellan was in charge of both the domestic services and military force of the castle. In application, as civilizations evolved beyond castles, the title continued as an administrative role as the leader of both civilian employees of a government as well a military unit.
The Castellan of the Cardassian Union is one application of this title, where the democratically selected Castellan is head of government, an executive administrator of both civilian government employees and the state military. (DS9 - Worlds of DS9 novel: Cardassia: The Lotus Flower)