The Danelaw (from the Old English Dena lagu) was an area of England under the administrative control of the Vikings (or Danes, or Norsemen) from the late 9th century. The term is also used to describe the set of legal terms and definitions established between Alfred the Great and the viking Guthrum which were set down in agreements such as the Treaty of Wedmore which established a modus vivendi between the Anglo-Saxons and the viking in-comers.
The area occupied by the Danelaw was roughly the area to the north of a line drawn between London and the river Mersey.