Historical region in the Southern half of Schleswig-Holstein
Holstein (German pronunciation: [ˈhɔlʃtaɪn] ; Northern Low Saxon: Holsteen; Danish: Holsten; Latin: Holsatia) is the region between the rivers Elbe and Eider. It is the southern half of Schleswig-Holstein, the northernmost state of Germany. Holstein once existed as the German County of Holstein (German: Grafschaft Holstein; 811–1474), the later Duchy of Holstein (German: Herzogtum Holstein; 1474–1866), and was the northernmost territory of the Holy Roman Empire. The history of Holstein is closely intertwined with the history of the Danish Duchy of Schleswig (Danish: Slesvig).
https://en.wikipedia.org/wiki/Holstein