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]

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



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ISTHMUS. A tongue or strip of land between two seas. Glos. on Law, 37, book
2, tit. 3, of the Dig.