A duke (male) (British English: or American English: ) or duchess (female) can either be a monarch ruling over a duchy or a member of the nobility, historically of highest rank below the monarch. The title comes from French duc, itself 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.

A duke (male) (British English: or American English: ) or duchess (female) can either be a monarch ruling over a duchy or a member of the nobility, historically of highest rank below the monarch. The title comes from French duc, itself 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.