watchword

noun2

1. •
   A word used as a motto, as expressive of a principle, belief, or rule of action; a rallying cry.

   • For-ward! be our watchword, Steps and voices joined; / Seek the things before us, Not a look behind. — 1871
2. •
   A prearranged reply to the challenge of a sentry or a guard; a password or signal by which friends can be known from enemies.

   • a Watchword sufficient for him that is wise — 1625