Ætheling was an Old English term (æþeling) used in Anglo-Saxon England to designate princes of the royal dynasty who were eligible for the kingship.
Edgar the Ætheling
Æthelred I was King of Wessex from 865 until his death in 871. He was the fourth of five sons of King Æthelwulf of Wessex, four of whom in turn became king. Æthelred succeeded his elder brother Æthelberht and was followed by his youngest brother, Alfred the Great. Æthelred had two sons, Æthelhelm and Æthelwold, who were passed over for the kingship on their father's death because they were still infants. Alfred was succeeded by his son, Edward the Elder, and Æthelwold unsuccessfully disputed the throne with him.
Æthelred as depicted in the early-fourteenth-century Genealogical Roll of the Kings of England
Charter S 332 dated 863 of King Æthelberht. Æthelred attests second from bottom on the left as "Æthelred fil[ius] reg[is]".
Charter S 338 dated 867. Æthelred, King of the West Saxons and the Men of Kent, grants Wighelm, priest, a seat in St Martin's Church, Canterbury, together with land. Most charters only survive as copies, and this is the only original of Æthelred to survive.