HOLSTEIN, formerly a duchy of Germany, now incorporated in Prussia. Until about 1 I 1 o the county of Holstein formed part of the duchy of Saxony, and it was made a duchy in 1472. From 146o to 1864 it was ruled by members of the house of Oldenburg, some of whom were also kings of Denmark. By the Treaty of Vienna (Aug. 1864) which concluded the Danish War, Holstein was ceded to Prussia and Austria jointly, but placed under the administration of Austria, which, however, was forced to deliver it over to Prussia after the Austro-Prussian War of 1866. (See