Definition of motto:
(n) :
a sentence, phrase, or word, forming part of an heraldic achievement; 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
"It was the motto of a bishop eminent for his piety and good works, ... Serve God, and be cheerful." -Addison.