Duke

Duke (or Duchess) is a hereditary title in the Wyneil Empire's royalty or nobility ranked below Prince and Grand Duke.  Dukes were once the rulers of the largest provinces, and remain the superiors of Counts, but have had their holdings and powers diminished with the elevation of some provinces into Grand Duchys.
A Duchy is the territory or geopolitical entity ruled by a Duke/Duchess, whereas the title and holdings are often called a dukedom.