Æthelfrith (ĕ`thəlfrĭth, ă–), d. 616, king of Northumbria (c.593–616). He was the first great Anglo-Saxon leader among the northern English; he united Bernicia and Deira into the kingdom of Northumbria. He repulsed an attack by the Scots in 603 and about 10 years later defeated the Welsh at Chester. During Æthelfrith's lifetime (if not solely as a result of the battle of Chester), the English penetrated to the Irish Sea, thus separating the Welsh in Wales from the Welsh in SW Scotland. Æthelfrith was killed in battle at the Idle River by Rædwald of East Anglia near the present-day town of Nottingham.