Deira (Old English: Derenrice or Dere) was a kingdom (559–664 AD) in Northern England that probably emerged when Anglian warriors conquered the Derwent Valley in the third quarter of the fifth century. It extended from the Humber to the Tees, and from the sea to the western edge of the Vale of York. It later merged with the kingdom of Bernicia, its northern neighbour, to form the kingdom of Northumbria.

The name of the kingdom is of Brythonic origin, perhaps from Deifr, meaning "waters", or from Daru, meaning "oak", in which case it would mean "the people of the Derwent", a derivation also found in the Latin name for Malton, Derventio.

According to Simeon of Durham (writing early in the 12th century), it extended from the Humber to the Tyne, but the land was waste north of the Tees. After the Brythonic kingdom centred on Eboracum, which may have been called Ebrauc, was taken by King Edwin, the city of Eboracum became its capital and was called Eoforwic ("boar-place") by the Angles.

The first Anglian king of Deira of whom we have any record is Ælla, who flourished in the later 6th century after conquering the realm from the Britons in 581. After his death, Deira was subject to king Æthelfrith of Bernicia, who united the two kingdoms into Northumbria. Æthelfrith ruled until the accession of Ælla's son Edwin, in 616 or 617, who also ruled both kingdoms until 633.

...
Wikipedia