The Collaborative International Dictionary of English v.0.48:

Shibboleth \Shib"bo*leth\, n. [Heb. shibb[=o]leth an ear of
   corn, or a stream, a flood.]
   1. A word which was made the criterion by which to
      distinguish the Ephraimites from the Gileadites. The
      Ephraimites, not being able to pronounce sh, called the
      word sibboleth. See --Judges xii.
      [1913 Webster]

            Without reprieve, adjudged to death,
            For want of well pronouncing shibboleth. --Milton.
      [1913 Webster] Also used in an extended sense.
      [1913 Webster]

            The th, with its twofold value, is . . . the
            shibboleth of foreigners.             --Earle.
      [1913 Webster]

   2. Hence, the criterion, test, or watchword of a party; a
      party cry or pet phrase.
      [1913 Webster]

WordNet (r) 3.0 (2006):

shibboleth
    n 1: a favorite saying of a sect or political group [syn:
         motto, slogan, catchword, shibboleth]
    2: a manner of speaking that is distinctive of a particular
       group of people