byword vs watchword

byword

noun
  • A proverb or proverbial expression, common saying; a frequently used word or phrase. 

  • A nickname or epithet. 

  • Someone or something that stands as an example (i.e. metonymically) for something else, by having some of that something's characteristic traits. 

  • A characteristic word or expression; a word or phrase associated with a person or group. 

  • An object of notoriety or contempt, scorn or derision. 

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 byword and watchword occurred in a corpus of books? (source: Google Ngram Viewer )