watchword

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A prearranged reply to a challenge, as from a guard or sentry; a password.
• noun A rallying cry.

from The Century Dictionary.

• noun A word or short phrase to be communicated on challenge to the watch or sentinels in a camp; a password or signal by which friends can be known from enemies.
• noun Hence Any preconcerted indication or a direction eagerly watched for, as a signal for action.
• noun A word used as a motto, as expressive of a principle or rule of action; a maxim, byword, or rallying-cry.
• noun The call of a watchman or sentry as he goes his rounds.

from the GNU version of the Collaborative International Dictionary of English.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a prearranged reply to the challenge of a sentry or a guard; a password or signal by which friends can be known from enemies
• noun a word used as a motto, as expressive of a principle or rule of action; a maxim, byword
• noun a rallying cry

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a secret word or phrase known only to a restricted group
• noun a slogan used to rally support for a cause