The Collaborative International Dictionary of English v.0.48:

Passport \Pass"port\, n. [F. passeport, orig., a permission to
   leave a port or to sail into it; passer to pass + port a
   port, harbor. See Pass, and Port a harbor.]
   1. Permission to pass; a document given by the competent
      officer of a state, permitting the person therein named to
      pass or travel from place to place, without molestation,
      by land or by water.
      [1913 Webster]

            Caution in granting passports to Ireland.
                                                  --Clarendon.
      [1913 Webster]

   2. A document carried by neutral merchant vessels in time of
      war, to certify their nationality and protect them from
      belligerents; a sea letter.
      [1913 Webster]

   3. A license granted in time of war for the removal of
      persons and effects from a hostile country; a
      safe-conduct. --Burrill.
      [1913 Webster]

   4. Figuratively: Anything which secures advancement and
      general acceptance. --Sir P. Sidney.
      [1913 Webster]

            His passport is his innocence and grace. --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

passport
    n 1: any authorization to pass or go somewhere; "the pass to
         visit had a strict time limit" [syn: pass, passport]
    2: a document issued by a country to a citizen allowing that
       person to travel abroad and re-enter the home country
    3: any quality or characteristic that gains a person a favorable
       reception or acceptance or admission; "her pleasant
       personality is already a recommendation"; "his wealth was not
       a passport into the exclusive circles of society" [syn:
       recommendation, passport]

The Devil's Dictionary (1881-1906):

PASSPORT, n.  A document treacherously inflicted upon a citizen going
abroad, exposing him as an alien and pointing him out for special
reprobation and outrage.