Ella has two origins. In Germanic languages it is a short form of names beginning with alja- (other, foreign) — names like Alia or Albertina. In Hebrew, Ella (אֵלָה) is a tree name, specifically the terebinth or oak, and appears as a place name in the Hebrew Bible.
Ella has been continuously used in English from the medieval period, popularised by Norman families. Its current high ranking owes much to Ella Fitzgerald ("the First Lady of Song") and to its sound: two syllables, one of the simplest names in English, with a soft -ah ending. It sits in the U.S. top 25.
Ella reduces to nine — the number of completion and quiet generosity.