The Collaborative International Dictionary of English v.0.48:

Slogan \Slo"gan\, n. [Gael. sluagh-ghairm, i.e., an army cry;
   sluagh army + gairm a call, calling.]
   The war cry, or gathering word, of a Highland clan in
   Scotland. --Sir W. Scott.
   [1913 Webster]

   2. Hence: A distinctive motto, phrase, or cry used by any
      person or party to express a purpose or ideal; a
      catchphrase; a rallying cry.
      [PJC]

WordNet (r) 3.0 (2006):

slogan
    n 1: a favorite saying of a sect or political group [syn:
         motto, slogan, catchword, shibboleth]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "slogan":
   bugle call, byword, call to arms, call-up, catch phrase, catchword,
   clarion, clarion call, cliche, conscription, cry, device, epigraph,
   epithet, exhortation, expression, fad word, go for broke, gung ho,
   idiom, inscription, levy, locution, mobilization, motto, muster,
   pet expression, phrase, rally, rallying cry, rebel yell,
   recruitment, shibboleth, tag line, trumpet call, vogue word,
   war cry, war whoop, watchword