Motto, n.; pl. Mottoes.
Etymology: [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.
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. It was the motto of a bishop eminent for his piety and good works, ... "Serve God, and be cheerful." Addison.
Shades of meaning
Fernald has no entry of its own for motto; he lists it under proverb.
Where he writes about it:
Precept is a command to duty; motto or maxim is a brief statement of cherished truth, the maxim being more uniformly and directly practical; "God is love" may be a motto, "Fear God and fear naught," a maxim. — from proverb