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]