Noble titles: Duke
A duke is a member of the nobility, historically of highest rank below the monarch, and historically controlling a duchy or a dukedom. The title comes from the Latin Dux Bellorum, which had the sense of "military commander" and was employed by both the Germanic peoples themselves and by the Roman authors covering them to refer to their war leaders.
In the Middle Ages the title signified first among the Germanic monarchies. Dukes were the rulers of the provinces and the superiors of the counts in the cities and later, in the feudal monarchies, the highest-ranking peers of the king.
In the Modern Age it mostly became a nominal rank without an actual principality. The notable exception would be the Grand Duchy of Luxembourg. It is still the highest titular peerage in France, Portugal, Spain, the United Kingdom, and Italy.
A woman who holds in her own right the title to such duchy or dukedom, or is the wife of a duke, is normally styled duchess. However, Queen Elizabeth II is known as Duke of Normandy in the Channel Islands and Duke of Lancaster in Lancashire.