In Britain the parallel form Alice, derived via Old French, has historically been more common than Adelaide, though this form did gain some currency in the 19th century due to the popularity of the German-born wife of King William IV, for whom the city of Adelaide in Australia was named in 1836.
The popularity of the name Eleanor in England during the Middle Ages was due to the fame of Eleanor of Aquitaine, as well as two queens of the following century: Eleanor of Provence, the wife of Henry III, and Eleanor of Castile, the wife of Edward I. More recently, it was borne by first lady Eleanor Roosevelt (1884-1962), the wife of American president Franklin Roosevelt.
In some cases this name can be a short form of longer names that start with Leo, such as Leonard.
The name was common in France during the Middle Ages, and was introduced from there to England, though it eventually became rare. Interest in it was revived by the character Melanie Wilkes from the novel Gone with the Wind (1936) and the subsequent movie adaptation (1939).