William Adelin (1103 - November 25, 1120) was the only legitimate son of Henry I of England. His early death in the White Ship disaster re-arranged the politics of England and France. (Though there is some remote possibility that he had a full brother who died on the White Ship.)
Since William died when he was 17 or 18, he did not have much impact on the political affairs of his father's domains. But he did figure into the events of the early 12th century in two ways.
During his long reign Henry would face several eruptions of hostilities with alliances of some of his neighbors. As part of an effort to detach Anjou, a long-time rival of Normandy, from such an alliance, in 1113 Henry betrothed William to Matilda, eldest daughter of count Fulk V of Anjou. The marriage itself finally took place in 1119.
The king of France was another of the hostile neighbors. A major item of contention there was the obstensible duty for Henry to do homage for Normandy. In 1115 Henry offered to have William do this in his stead, and this offer was eventually accepted in 1120, after an intervening period of war. William did homage to Louis VI of France sometime in the middle of 1120. For this reason William is sometimes counted as duke of Normandy.
William's mother Queen Matilda usually served as Henry's regent in England while he was away in Normandy. After her death in 1118 William was old enough to serve in her stead. He was closely advised in this role by the king's administrators such as Roger of Salisbury. During the last year or so of his life he was sometimes referred to as rex designatus (king designate). Nevertheless William had very little real power.
The second part of his name is variously referred to as Audelin, Atheling, or Aetheling. In any case it is derived from the Old English Ætheling, meaning "son of the king".
