Northumbria was an Anglo-Saxon kingdom in the north of what is now England and the south of what is now Scotland. From the seventh to the ninth centuries it was powerful — kings, churches, and famous books. The name is a Latin-style form of an Old English name, Norþhymbre / Norþanhymbre, meaning the people north of the River Humber: North + (of the) Humber. At its strongest the kingdom was far bigger than today’s Northumberland; it reached down through Durham and into Yorkshire. Viking attacks later broke its power. Though actually, people still use Northumbria unofficially to mean Northumberland, or the wider northeast, especially on signs, in tourist writing, and in names like a university. If you are talking about history, Northumbria is the kingdom. If you need the county on a modern map, you want Northumberland.