Duke

(Fr. Duc, It. Duca, Sp. Duque, Port. Duque, Ger. Herzog)

The highest rank in the peerage of Great Britain. The name is derived from the Latin word dux, by which term, under the later Emperors, the Governor of a province was designated. The first who bore this title was the Governor of the Marchia Rhætica. When nearly all the institutions of the Roman empire were swept away by the different barbarous nations that obtained possession of it, it was in Gaul only that the title was still preserved.
From being a frequent dignity during the earlier period of the Frank dynasty, it became restricted under the second race of Kings to only two or three of the most powerful peers who effected and obtained power independently of the Crown. From these, no doubt, the title was borrowed by the Sovereigns of some minor independent states, such as Lorraine and Tuscany; the latter, however, assuming the more distinguished appellation of Grand-Duke.
According to Camden, during  Saxons times,  the name of Duke was employed in this England in its original signification of commander of an army. After the Conquest it was entire disused for some time, partly because the Normans introduced their own titles in their own language, and partly, perhaps, because the Conqueror’s own title as Sovereign of Normandy was that of Duke. It was not revived until the reign of Edward III. who created his son, Edward the Black Prince, Duke of Cornwall in 1337.

See Nobility.