(Russian, derived from Germanic *kuningaz, kunungaz, "king")
A title used for Russian rulers and princes, used in varying forms in the different Slavic languages.1
At the time of the formation of Russia, the word knyaz was applied to the local tribal chieftains/princes, and later to the Rurikid dynasty, which replaced them. In time, the title became an attribute of the dynasty, and not of a ruler of a particular geographic location.
From the 10th century, the title velikiye knyaz ("great prince") would seem to have been in use to describe the formal superior of all the Russian princes, in Kiev. From the 15th century, the title of knyaz was gradually supplanted by the title of czar. Later, knyaz was applied to members of foreign princely houses who had entered Russian service - particularly those from Lithuania.2
By the 18th century, the title had become watered down to the point where even non-princely individuals might have it applied to them.
1 The Wends, for instance, use knees or knies, instead. From the middle ages until the 20th century, the title of knees of the Wends was subsumed into the titulary of the Danish monarchs, as "de Venders". Today, it is considered obsolete, and is no longer used.
2 As a result of the expansion of Russian hegemony in the 13th and 14th centuries, the Lithuanian princely dynasty had begun using the title of knyaz, too.