The Collaborative International Dictionary of English v.0.48:

Archduke \Arch`duke"\, n. [Pref. arch- + duke.]
   A prince of the imperial family of Austria.
   [1913 Webster]

   Note: Formerly this title was assumed by the rulers of
         Lorraine, Brabant, Austria, etc. It is now appropriated
         to the descendants of the imperial family of Austria
         through the make line, all such male descendants being
         styled archduke, and all such female descendants
         archduchesses.
         [1913 Webster]

WordNet (r) 3.0 (2006):

archduke
    n 1: a sovereign prince of the former ruling house of Austria