A grand duke is the male ruler of a sovereign monarchy called a grand duchy (such as Luxemburg) or the head of an autonomous dependency of a monarchy (such as Finland prior to 1917). It is also used to describe the head some confederated states within the Holy Roman Empire. The title is limited almost exlusively to Slavic and German countries. Sovereign grand dukes are considered to be higher in honor than sovereign princes.
Grand Duke is also the translated title given to brothers, sons, and grandsons of the Tsar of Russia. When referring to Russian royalty, a more accurate translation of the this title is Great Prince, especially in the pre-Petrine era, but the term is not standard or widely used.
Finally, Grand Duke is the translated form of the title megadoukas infrequently used in the Byzantine Empire during the Paleologus dynasty.
Grand Duchess is the feminine form of this title.
The term is derived from grand + duke and probably originated in its original form from Germany.