grand duchess

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The wife or widow of a grand duke.
• noun A woman who is the sovereign of a grand duchy.
• noun The daughter or granddaughter of a czar in the male line.
• noun Used as the title for such a noblewoman.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The wife or widow of a grand duke.
• noun incorrectly sometimes applied to render a grand prince's female spouse
• noun A daughter or granddaughter of a sovereign (czar or later emperor) of the Russian Empire or a pretender to the throne thereof.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the wife of a grand duke or a woman holding that rank in her own right