Watchword

Webster's Dictionary of the English Language

·noun A sentiment or motto; ·esp., one used as a rallying cry or a signal for action.

II. Watchword ·noun A word given to sentinels, and to such as have occasion to visit the guards, used as a signal by which a friend is known from an enemy, or a person who has a right to pass the watch from one who has not; a countersign; a password.