The Old English word Æþeling means 'child of a noble family' - prince, in effect. Modern historians often use the term 'throne-worthy' to refer to those athelings of the royal house who were eligible for kingship. There was no automatic rule of primogeniture - a king's successor did not need to be his eldest son, and indeed was not necessarily his son at all. Thus, in the 9th century, King Athelwulf had five sons, four of whom became king in turn: Athelstan (d. c.850, predeceasing his father), King Athelbald (855-860), King Athelbert (860-866), King Athelred (866-871) and finally King Alfred (871-899).

Athelred (d. 871) had three sons, all of whom were too young to inherit when he died. Two of them had died before their uncle King Alfred, but the third, Athelwald, disputed the claim of Alfred's son, Edward, to the throne, and led a rebellion against him.
A coin with the name Alwaldus, found in Yorkshire and possibly minted for Athelwald.

There is a fascinating article here by Anglo-Saxonist, fantasy novelist and Tolkien scholar, Professor Tom Shippey, on the family politics of Beowulf, compared in particular with the West Saxon royal family in the generations after King Alfred.