grand duke

the sovereign of a territory called a grand duchy, ranking next below a king.

a son of a czar or of a czar's son.

Origin of grand duke

First recorded in 1685–95

grand-du·cal, adjective

But civil disorders followed, and in 1849 the grand-duke returned under an Austrian escort.

Encyclopaedia Britannica, 11th Edition, Volume 10, Slice 5 |Various
However, his title of grand-duke was confirmed, and as grand-duke of Hesse and of the Rhine he entered the Germanic confederation.

Encyclopaedia Britannica, 11th Edition, Volume 13, Slice 4 |Various
It is the residence of the grand-duke and the seat of government of the duchy.

Encyclopaedia Britannica, 11th Edition, Volume 7, Slice 9 |Various
"Sire, he is about as tall as the Grand-duke of Berg," said Duroc.

Napoleon and the Queen of Prussia |L. Mhlbach,

Basil was the first grand-duke of Moscow who adopted the title of tsar and the double-headed eagle of the East Roman empire.

Encyclopaedia Britannica, 11th Edition, Volume 3, Part 1, Slice 3 |Various

grand duke

a prince or nobleman who rules a territory, state, or principality

a son or a male descendant in the male line of a Russian tsar

a medieval Russian prince who ruled over other princes