Westphalia (in German, Westfalen) is a (historic) region in Germany, centred on the cities of Dortmund, Münster, Bielefeld, and Osnabrück and now included in the Bundesland of North Rhine-Westphalia (and the (south-)west of Lower Saxony).
Westphalia is roughly the region between the rivers Rhine and Weser, located north of the Ruhr river. No exact definition of borders can be given, because the name "Westphalia" was applied to several different entities in history. For this reason specifications of area and population are greatly differing. They range between 16,000 and 22,000 km² resp. between 4.3 million and 8 million inhabitants.
Originally Westphalia was a part of the duchy of Saxony, until it was elevated to the rank of a duchy by emperor Barbarossa in 1180. This duchy comprised only a small area south of the Lippe River.
There was a kingdom of Westphalia from 1807 to 1813. It was founded by Napoleon and was a French vassal state. This state only shared the name with the historical region, it contained mostly Hessian and Eastphalian regions and only a relatively small part of Westphalia.
Afterwards Westphalia became a province of Prussia. The northernmost portions of the former Westphalia, including the town of Osnabrück, were ceded to the states of Hanover and Oldenburg.
The present state of North Rhine-Westphalia is composed of the former Prussian province of Westphalia, the northern half of the former Prussian Rhine province, and the former state of Lippe.
Westphalia is known for the 1648 Peace of Westphalia (in fact the two treaties of Münster and Osnabrück), which ended the Thirty Years' War.
A linguistic definition of Westphalia includes the former Prussian province (except the Siegerland) and the region around Osnabrück. Present-day common use, however, restricts the notion to the present northern part of North Rhine-Westphalia, because of the name.
Map of Germany centred on region (http://www.multimap.com/map/browse.cgi?client=public&X=800000&Y=6700000.93007549&width=700&height=400&gride=830841&gridn=6678902.93007549&srec=0&coordsys=mercator&db=DE&addr1=&addr2=&addr3=Dortmund&pc=&advanced=&local=&localinfosel=&kw=&inmap=&table=&ovtype=&zm=1&scale=10000000)