Kniaz
Kniaz (Russian князь (knyaz)) is a title from the history of Russia. It is usually translated into English either as Prince or Duke, although the correspondence is not exact. The etymology is probably related to German KÃÂönig. The meaning was changing during the history.
Initially it was used to denote the chieftain of a tribe. Later, with the development of feudal statehood it become the title of a ruler of a state (княжество (kniazhestvo), translated as Duchy, although Princedom would be more precise), e.g., of Kievan Rus'. In such cases the title is better translated as Prince.
As the degree of centalization grew, the ruler acquired the title Velikii Kniaz (translated as Grand duke, see Russian Grand Dukes). he ruled a Velikoe Knyazhestvo (Grand Duchy), while a ruler of its vassal constituent (udel, udelnoe kniazhestvo) was called udelny kniaz or simply kniaz.
Eventually Velikii Kniaz Ivan IV of Russia in 1547 was crowned as Tsar, and tsars become Emperors, and the title Velikii Kniaz has been applied to sons and grandsons (through male lines) of the Tsars and Emperors of Russia, see Titles for Tsar's family.
The title Kniaz was widely used on the area of Grand Duchy of Lithuania.
At the same time, Kniaz has become a hereditary title of nobility granted by Tsar. The title kniaz (in this case it corresponds to Duke) usually came with udel (land, property), i.e., the term Duchy will be a close match. Unusually many of "kniazes" were in the lands of Caucasus.
Finally, within the Russian Empire of 1809-1917, Finland was called Grand Duchy of Finland (Velikoe Kniazhestvo Finlandskoe).