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.
Because Elijah was a popular figure in medieval tales, and because his name was borne by a few early saints (who are usually known by the Latin form Elias), the name came into general use during the Middle Ages. In medieval England it was usually spelled Elis. It died out there by the 16th century, but it was revived by the Puritans in the form Elijah after the Protestant Reformation. The name became popular during the 1990s and 2000s, especially in America where it broke into the top ten in 2016.
In some cases this name can be a short form of longer names that start with Leo, such as Leonard.