Latinized form of the name of the Anglo-Saxon kingdom of Norðhymbre, which lay north of the river Humber (Latin Humbri fluminis, c. 720), an ancient pre-English river name of unknown origin. It was the leading power of England during part of the 7c. and 8c. Related: Northumbrian. The Northumbrians seem at times to have referred to the Mercians as Southumbrians. The English county name of Northumberland is attested from c. 1200 (North-humbre-lond).

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Definitions of Northumbria from WordNet

Northumbria (n.)
an Anglo-Saxon kingdom in northern England until 876;