Noble or royal title in some European countries and their colonies
Duke is a male title either of a monarch ruling over a duchy, or of a member of royalty, or nobility. As rulers, dukes are ranked below emperors, kings, grand princes, grand dukes, and above sovereign princes. As royalty or nobility, they are ranked below grand dukes and above or below princes, depending on the country or specific title.
https://en.wikipedia.org/wiki/Duke