Source : Webster's Revised Unabridged Dictionary (1913)

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.

   {Isthmus of the fauces}. (Anat.) See {Fauces}.

Source : WordNet®

isthmus
     n 1: a relatively narrow strip of land (with water on both sides)
          connecting two larger land areas
     2: a narrow band of tissue connecting two larger parts of an
        anatomical structure
     [also: {isthmi} (pl)]