grand duchess 
noun
1The wife or widow of a grand duke.Instead I shall be the most sought after girl and then when Fabian is old enough I will become his grand duchess....
- Henri, Grand Duke of Luxembourg, and his consort, grand duchess Maria Teresa, are arriving on a three-day state visit to Bulgaria on October 11.
1.1A woman holding the rank of grand duke in her own right.Your wife will be grand duchess of all Letzenstein....
- Since Charlotte's abdication, the grand duchy has been ruled again only by men, and the sisters remain the only two reigning grand duchesses of Luxembourg in history.
- Of all the children, only she would achieve a notable career, and Orazio was clearly active in furthering it, as we learn also from his well-known letter of 1012 to the grand duchess of Tuscany in praise of Artemisia's skills as an artist.
2 historical A daughter (or granddaughter) of a Russian tsar.One of Queen Victoria's many grandchildren, Queen Marie of Romania was born in Kent in 1875, and always thought of England as home; her father was the Duke of Edinburgh, and her mother a Russian grand duchess....
- I am the grand duchess Alexandra Romanov, though I'm sure you already knew that.
- Ninotchka is a Russian commissar who has come to Paris to sell jewellery belonging to a grand duchess.