The Collaborative International Dictionary of English v.0.48:

Pythian \Pyth"i*an\, a. [L. Pythius, Gr. ? belonging to Pytho,
   the older name of Delphi and its environs: cf. F. pythien.]
   Of or pertaining to Delphi, to the temple of Apollo, or to
   the priestess of Apollo, who delivered oracles at Delphi.
   [1913 Webster]

   Pythian games (Gr. Antiq.), one of the four great national
      festivals of ancient Greece, celebrated near Delphi, in
      honor of Apollo, the conqueror of the dragon Python, at
      first once in eight years, afterward once in four.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Pythian Games
    n 1: the ancient Panhellenic celebration at Delphi held every
         four years in the third year of the Olympiad in honor of
         Apollo