Northumbria is primarily the name of an Anglian kingdom which was formed in Great Britain at the beginning of the 7th century, and of the much smaller earldom which succeeded the kingdom. The name reflects that of the southern limit to the kingdom's territory, which was the River Humber, and in the 12th century writings of Henry of Huntingdon the kingdom was defined as one of the Heptarchy of Anglo-Saxon kingdoms.
At its greatest the kingdom extended from the Humber to the Forth. The later earldom was bounded by the River Tees in the south and the River Tweed in the north (broadly similar to the modern North East England) and was recognised as part of England by the Anglo-Scottish Treaty of York in 1237. Berwick-upon-Tweed, which is north of the Tweed, was defined as subject to the laws of England by the Wales and Berwick Act of 1746 and is administered now as a part of the county of Northumberland and the region of North East England.