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]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "isthmus":
   abbreviation, astriction, astringency, bottleneck, canal, cervix,
   channel, circumscription, coarctation, compactedness, compaction,
   compression, compressure, concentration, condensation,
   consolidation, constriction, constringency, contraction,
   contracture, curtailment, decrease, defile, diminuendo, hourglass,
   hourglass figure, knitting, narrow, narrow place, narrowing,
   narrows, neck, pass, puckering, pursing, reduction, shortening,
   solidification, strait, stranglement, strangulation, striction,
   stricture, systole, throat, wasp waist, wrinkling