WATCHWORD
password, watchword, word, parole, countersign
(noun) a secret word or phrase known only to a restricted group; “he forgot the password”
Source: WordNet® 3.1
Etymology
Noun
watchword (plural watchwords)
A word used as a motto, as expressive of a principle, belief, or rule of action; a rallying cry.
Synonyms: catchword, byword, maxim, mantra, motto
(archaic) A prearranged reply to the challenge of a sentry or a guard; a password or signal by which friends can be known from enemies.
Synonyms: passphrase, password, underword
Source: Wiktionary
Watch"word`, n.
1. 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.
2. A sentiment or motto; esp., one used as a rallying cry or a signal
for action.
Nor deal in watchwords overmuch. Tennyson.
Source: Webster’s Unabridged Dictionary 1913 Edition