From WordNet (r) 3.0 (2006) [wn]:
motto
n 1: a favorite saying of a sect or political group [syn:
motto, slogan, catchword, shibboleth]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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.
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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]