The Collaborative International Dictionary of English v.0.48:

Belle \Belle\ (b[e^]l), n. [F. belle, fem. of bel, beau,
   beautiful, fine. See Beau.]
   A young lady of superior beauty and attractions; a handsome
   lady, or one who attracts notice in society; a fair lady.
   [1913 Webster]

WordNet (r) 3.0 (2006):

belle
    n 1: a young woman who is the most charming and beautiful of
         several rivals; "she was the belle of the ball"