grand duke 
noun
1A prince or nobleman ruling over a territory in certain European countries.The official guest list named at least 70 kings, queens, grand dukes, princes, counts and lesser nobles....
- Whether forced or voluntary, Roman emperors, kings and queens, hereditary princes and grand dukes and, yes, even popes have abdicated.
- The grand duke of Tuscany, Ferdinand I, in the meantime, showed a new level of independence from Spain by marrying a French princess, and announcing the marriage of his niece Maria to the French king.
2 historical A son (or grandson) of a Russian tsar.Two of his daughters married Russian grand dukes; one became Queen of Italy; another came into the orbit of British royalty by marrying a Battenberg prince....
- Close relatives of the tsar were called grand princes, usually translated as grand dukes.
- Destinations outside the city worth adding to your trip include the Kolomenskoe Estate, overlooking the Moskva river, where grand dukes and tsars have spent their summers since the 14th century.