A duke (male) or duchess (female) is a member of the nobility, historically of highest rank below the monarch, and historically controlling a duchy. The title comes from the Latin Dux, 'leader', a term used in republican Rome to refer to a military commander without an official rank (particularly one of Germanic or Celtic origin), and later coming to mean the leading military commander of a province.
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.
During the 19th century many of the smaller German and Italian states were ruled by Dukes or Grand Dukes. At present however, with the exception of the Grand Duchy of Luxembourg, there are no dukes who rule. Duke remains the highest titular peerage in France, Italy, Portugal, Spain, and the United Kingdom. The Pope, as a temporal sovereign, has also but rarely granted the title of Duke and Duchess to persons for services to the Holy See.
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.