The Collaborative International Dictionary of English v.0.48:

Isthmian \Isth"mi*an\, a. [L. Isthmius, Gr. ?. See Isthmus.]
   Of or pertaining to an isthmus, especially to the Isthmus of
   Corinth, in Greece.
   [1913 Webster]

   Isthmian games (Gr. Antiq.), one of the four great national
      festivals of Greece, celebrated on the Isthmus of Corinth
      in the spring of every alternate year. They consisted of
      all kinds of athletic sports, wrestling, boxing, racing on
      foot and in chariots, and also contests in music and
      poetry. The prize was a garland of pine leaves.
      [1913 Webster]

WordNet (r) 3.0 (2006):

isthmian
    adj 1: of or relating to or forming an isthmus