Eastphalia (Ostfalen; Eastphalian: Oostfalen) is a historical region in northern Germany, encompassing the eastern Gaue (shires) of the historic stem duchy of Saxony, roughly confined by the River Leine in the west and the Elbe and Saale in the east. The territory corresponds with the southeastern part of the present-day states of Lower Saxony, western Saxony-Anhalt and northern Thuringia. Together with Westphalia, central Angria and Nordalbingia it was one of the four main Saxon administrative regions.
The name Ostfalen probably means "east plain". Falen is a Germanic word meaning "flat", "level" and "plain" and is related to the Old Swedish word fala, found today in place names such as Falbygden and Falun, as well as in the northern German town of Fallstedt. The North German Plain of Eastphalia and Westphalia, divided by the Weser river, stands in contrast to the hilly region to the south, the Central Uplands of Franconia and Thuringia. Unlike the name Westphalia, which was perpetuated by the Duchy of Westphalia held by the archbishop-electors of Cologne, the name Eastphalia gradually fell out of use when the Saxon stem duchy dissolved upon the deposition of Duke Henry the Lion in 1180.
German linguists reintroduced the term in the 19th century in the course of researching the Eastphalian language as a West Low German dialect. With the rise of racial studies in the 19th and early 20th century, mention began to be made – notably by Hans F. K. Günther – of a "Phalian" subtype (fälische Rasse) of an "Aryan race" according to the occult concept of Helena Blavatsky and her Ariosophic followers.
With Charlemagne's defeat and baptism of Duke Widukind in 785 during the Saxon Wars, his lands were integrated into the Frankish Empire and the Saxons were increasingly converted to Christianity. The bishoprics of Halberstadt and Hildesheim were established in eastern Saxony, bounded by the Oker river, in 804 and 815 respectively. The medieval Duchy of Saxony was divided between the districts of Eastphalia (Ostfalahi), Westphalia and Engern.