The Collaborative International Dictionary of English v.0.48:

Isthmus \Isth"mus\ (?; 277), n.; pl. Isthmuses. [L. isthmus,
   Gr. 'isqmo`s a neck, a neck of land between two seas, an
   isthmus, especially the Isthmus of Corinth; prob. from the
   root of 'ie`nai to go; cf. Icel. ei[eth] isthmus. See
   Issue.] (Geog.)
   A neck or narrow slip of land by which two continents are
   connected, or by which a peninsula is united to the mainland;
   as, the Isthmus of Panama; the Isthmus of Suez, etc.
   [1913 Webster]

   Isthmus of the fauces. (Anat.) See Fauces.
      [1913 Webster]

WordNet (r) 3.0 (2006):

isthmus
    n 1: a relatively narrow strip of land (with water on both
         sides) connecting two larger land areas
    2: a cord-like tissue connecting two larger parts of an
       anatomical structure [syn: isthmus, band]