The title of Archduke (feminine: Archduchess) (German: Erzherzog, feminine form: Erzherzogin) denotes a noble rank above Duke and below King, used only by princes of the Houses of Habsburg and Habsburg-Lorraine.
Territory ruled by an archduke or archduchess is called an archduchy.
Read more about Archduke: Address, Ruler Style, Other Dynastic Habsburg Use, Insignia, Fictional Archdukes, Sources and References