Archduke

The title of Archduke (feminine: Archduchess) (German: "Erzherzog", feminine form: "Erzherzogin") is a rank above Duke and under King. It is very rare. It was used only by males of the House of Habsburg. 
Ruler style.
The English word is recorded only since 1530, derived from Middle Latin "archidux", from "arch(i)-" (primary or chief) + "dux" 'duke' .
Archduke ("Erzherzog") is a title distinct from Grand Duke ("Großherzog" or "Großfürst") that is used in some other German royal houses and still in sovereign Luxemburg.