Merriam-Webster Online defines motto in this way:

Main Entry: mot·to
Pronunciation: 'mä-(")tO
Function: noun
Inflected Form(s): plural mottoes also mottos
Etymology: Italian, from Late Latin muttum grunt, from Latin muttire to mutter
1 : a sentence, phrase, or word inscribed on something as appropriate to or indicative of its character or use
2 : a short expression of a guiding principle