Balkans
In the 19th century, the Serbian term knez (кнез) and the Bulgarian term knyaz (княз) were revived to denote semi-independent rulers of those countries, such as Alexander Karađorđević and Alexander of Battenberg. Prior to Battenberg, the title knyaz was born by Simeon I during the First Bulgarian Empire (9th-10th century). At the height of his power, Simeon adopted the title of tsar ("emperor"), as did the Bulgarian rulers after the country became officially independent in 1908.
As of Bulgaria's independence in 1908, Knyaz Ferdinand became Tsar Ferdinand, and the words knyaz/knyaginya began to be used instead for the tsar's children – the heir to the throne, for example, held the title Knyaz Tarnovski ("Knyaz of Tarnovo").
In parts of Serbia and western Bulgaria, knez was the informal title of the elder or mayor of a village or zadruga until around the 19th century. Those are officially called gradonačelnik (Serbia) and градоначалник (gradonachalnik) or кмет (kmet) (Bulgaria).
Read more about this topic: Knyaz