A chief or ruler, after Roman usage, which referred to a military commander as dux. Now it is the highest rank of hereditary title of nobility, next below a prince (who is next below the monarch). The title was used in France, e.g. William I was duke of Normandy. The first English duke in this sense was Edward the Black Prince, created duke of Cornwall in 1337. The creation of the title was part of an imitation of the French court and its elaborate peerage. Four more dukes were created by Edward III, all members of the royal family. By the beginning of the 16c there were some 45 dukes. [< Lat. dux = leader] -
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.