Knyaz or knez is a historical Slavic title, used both as a royal and noble title, usually translated into English either as Prince or less commonly as Duke, and in Latin sources as comes or princeps, but the word was originally derived from the Proto-Germanic kuningaz (king).
The female form transliterated from Bulgarian and Russian is knyaginya (княгиня), kniahynia (княгиня) in Ukrainian, kneginja in Slovene, Croatian and Serbian (Serbian Cyrillic: кнегиња). In Russian, the daughter of a knyaz is knyazhna (княжна), in Ukrainian is kniazivna (князівна). In Russian, the son of a knyaz is knyazhich (княжич) (old form).
The title is pronounced and written similarly in different European languages. In Croatian, Bosnian and West Slavic languages, such as Polish, the word has later come to denote "lord", and in Czech, Polish and Slovak also came to mean "priest" (kněz, ksiądz, kňaz) as well as "duke" (knez, kníže, książę, knieža). In Sorbian it means simply "Mister". Today the term knez is still used as the most common translation of "prince" in Bosnian, Croatian and Serbian literature. "Knez" is also found as a surname in former Yugoslavia.(Wikipedia)