The Duchy of Prussia (, ) or Ducal Prussia (German: Herzogliches Preußen, Polish: Prusy Ksiazece) was a duchy in eastern Prussia established during the Protestant Reformation in 1525. It was the first Lutheran duchy with a dominant German-speaking population, as well as Polish and Lithuanian minorities. In old texts and in Latin, the term Prut(h)enia refers alike to Ducal Prussia, its western neighbor Royal Prussia, and their common predecessor, Teutonic Prussia. The adjectival form of the name was "Prut(h)enic".