The Collaborative International Dictionary of English v.0.48:

Motto \Mot"to\, n.; pl. Mottoes. [It. motto a word, a saying,
   L. muttum a mutter, a grunt, cf. muttire, mutire, to mutter,
   mumble; prob. of imitative origin. Cf. Mot a word.]
   1. (Her.) A sentence, phrase, or word, forming part of an
      heraldic achievment.
      [1913 Webster]

   2. A sentence, phrase, or word, prefixed to an essay,
      discourse, chapter, canto, or the like, suggestive of its
      subject matter; a short, suggestive expression of a
      guiding principle; a maxim.
      [1913 Webster]

            It was the motto of a bishop eminent for his piety
            and good works, . . . "Serve God, and be cheerful."
                                                  --Addison.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

187 Moby Thesaurus words for "motto":
   achievement, adage, alerion, ana, analects, animal charge, annulet,
   aphorism, apophthegm, apothegm, argent, armorial bearings, armory,
   arms, axiom, azure, bandeau, banner, banner head, bar,
   bar sinister, baton, battle cry, bearings, bend, bend sinister,
   billet, blazon, blazonry, bordure, broad arrow, byword,
   cadency mark, canton, caption, catchword, chaplet, charge, chevron,
   chief, coat of arms, cockatrice, collected sayings, coronet,
   crescent, crest, cross, cross moline, crown, cry, current saying,
   device, dictate, dictum, difference, differencing, distich,
   drop head, dropline, eagle, epigram, epigraph, epithet, ermine,
   ermines, erminites, erminois, escutcheon, expression, falcon, fess,
   fess point, field, file, flanch, fleur-de-lis, fret, fur, fusil,
   garland, gnome, golden saying, griffin, guide, gules, gyron,
   hanger, hatchment, head, heading, headline, helmet,
   heraldic device, honor point, impalement, impaling, inescutcheon,
   inscription, jump head, label, legend, lion, lozenge, mantling,
   marshaling, martlet, mascle, maxim, metal, moral, mot, mullet,
   nombril point, octofoil, or, oracle, ordinary, orle, overline,
   pale, paly, pean, pheon, phrase, pithy saying, precept, prescript,
   principle, proverb, proverbial saying, proverbs, purpure, quarter,
   quartering, rallying cry, rose, rubric, rule, running head,
   running title, sable, saltire, saw, saying, scarehead, screamer,
   scutcheon, sentence, sententious expression, shibboleth, shield,
   slogan, sloka, spread, spread eagle, spreadhead, stock saying,
   streamer, subhead, subheading, subordinary, subtitle,
   superscription, sutra, tag line, teaching, tenne, text, tincture,
   title, title page, torse, tressure, unicorn, vair, verse, vert,
   war cry, watchword, wisdom, wisdom literature, wise saying,
   witticism, word, words of wisdom, wreath, yale