The Anglian territory of Bernicia was approximately equivalent to the modern English counties of Northumberland and Durham, and the Scottish counties of Berwickshire and East Lothian, stretching from the Forth to the Tees. In the early 7th century, it merged with its southern neighbour, Deira, to form the kingdom of Northumbria and its borders subsequently expanded considerably.
Bernicia occurs in Old Welsh poetry as Bryneich or Brynaich and in the 9th-century Historia Brittonum, (§ 61) as Berneich or Birneich. This was most likely the name of the native Brittonic kingdom, whose name was then adopted by the Anglian settlers who rendered it in Old English as Bernice or Beornice. The counter hypotheses suggesting these names represent a Welsh adaption of an earlier English form remains unsupported, as it raises more questions as to why Brythonic would need a new foreign name for the area.
Local linguistic evidence suggests continued political activity in the area before the arrival of the Angles. Important Anglian centres in Bernicia bear names of British origin, or are known by British names elsewhere: Bamburgh is called Din Guaire in the Historia Brittonum; Dunbar (where Saint Wilfrid was once imprisoned) represents Dinbaer; and the name of Coldingham is given by Bede as Coludi urbs ("town of Colud"), where Colud seems to represent the British form, possibly for the hill-fort of St Abb's Head.
Analysis of a potential derivation has not produced a consensus. The most commonly cited etymology gives the meaning as "Land of the Mountain Passes" or "Land of the Gaps" (tentatively proposed by Kenneth H. Jackson). An earlier derivation from the tribal name of the Brigantes has been dismissed as linguistically unsound. In 1997 John T. Koch suggested the conflation of a probable primary form *Bernech with the native form *Brïγent for the old civitas Brigantum as a result of Anglian expansion in that territory during the 7th century.
The Brythonic kingdom of the area was formed from what had once been the southern lands of the Votadini, possibly as part of the division of a supposed 'great northern realm' of Coel Hen in c. AD 420. This northern realm is referred to by Welsh scholars as Yr Hen Ogledd or, literally, "The Old North". The kingdom may have been ruled from the site that later became the English Bamburgh, which certainly features in Welsh sources as Din Guardi. Near this high-status residence lay the island of Lindisfarne (formerly known, in Welsh, as Ynys Metcaut), which became the seat of the Bernician bishops. It is unknown when the Angles finally conquered the whole region, but around 604 is likely.