The Collaborative International Dictionary of English v.0.48:

Sibyl \Sib"yl\, n. [L. sibylla, Gr. ????.]
   1. (Class. Antiq.) A woman supposed to be endowed with a
      spirit of prophecy.
      [1913 Webster]

   Note: The number of the sibyls is variously stated by
         different authors; but the opinion of Varro, that there
         were ten, is generally adopted. They dwelt in various
         parts of Persia, Greece, and Italy.
         [1913 Webster]

   2. A female fortune teller; a pythoness; a prophetess. "An
      old highland sibyl." --Sir W. Scott.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sibyl
    n 1: a woman who tells fortunes
    2: (ancient Rome) a woman who was regarded as an oracle or
       prophet