motto vs watchword

motto

noun
  • 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. 

  • A sentence, phrase, or word, forming part of an heraldic achievement. 

watchword

noun
  • A word used as a motto, as expressive of a principle, belief, or rule of action; a rallying cry. 

  • A prearranged reply to the challenge of a sentry or a guard; a password or signal by which friends can be known from enemies. 

How often have the words motto and watchword occurred in a corpus of books? (source: Google Ngram Viewer )