"Deifr", meaning "waters") was a kingdom in England
during the 6th century
AD. It later merged with the kingdom of Bernicia
(Brythonic, "Brynaich") to form the kingdom of Northumbria
According to Simeon of Durham it extended from the Humber to the Tyne, but the land was waste north of the Tees. York was the capital of its kings.
The date of its first settlement is quite unknown, but the first king of whom we have any record is Ælla. After his death Deira was subject to king Æthelfrith of Northumbria until the accession of Ælla's son Edwin, in 616 or 617, who ruled both kingdoms till 633.
Osric, the nephew of Edwin, ruled Deira (633?634), but his son Oswine was put to death by Oswiu in 651. For a few years subsequently Deira was governed by Aethelwald son of Oswald.
Bede wrote of Deira in his Historia Ecclesiastica.