The Collaborative International Dictionary of English v.0.48:

Watchword \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.
      [1913 Webster]

   2. A sentiment or motto; esp., one used as a rallying cry or
      a signal for action.
      [1913 Webster]

            Nor deal in watchwords overmuch.      --Tennyson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

watchword
    n 1: a slogan used to rally support for a cause; "a cry to
         arms"; "our watchword will be `democracy'" [syn: war cry,
         rallying cry, battle cry, cry, watchword]
    2: a secret word or phrase known only to a restricted group; "he
       forgot the password" [syn: password, watchword, word,
       parole, countersign]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "watchword":
   bugle call, byword, call to arms, call-up, catchword, clarion,
   clarion call, conscription, countersign, device, epigraph, epithet,
   exhortation, go for broke, gung ho, inscription, levy,
   mobilization, motto, muster, open sesame, password, phrase, rally,
   rallying cry, rebel yell, recruitment, secret grip, shibboleth,
   slogan, tag line, tessera, token, trumpet call, war cry, war whoop,
   word