The Collaborative International Dictionary of English v.0.48:

Pythoness \Pyth"o*ness\, n. [L. pythonissa: cf. F. pythonisse.
   See Pythian.]
   1. (Gr. Antiq.) The priestess who gave oracular answers at
      Delphi in Greece.
      [1913 Webster]

   2. Any woman supposed to have a spirit of divination; a sort
      of witch. --Bp. Hall.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pythoness
    n 1: a witch with powers of divination
    2: (Greek mythology) the priestess of Apollo at Delphi who
       transmitted the oracles [syn: Pythia, Pythoness]