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]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

PASSPORT, SEA BRIEF, or SEA LETTER, maritime law. A paper containing a
permission from the neutral state to the captain or master of a ship or
vessel to proceed on the voyage proposed; it usually contains his name and
residence; the name, property, description, tonnage and destination of the
ship; the nature and quantity of the cargo; the place from whence it comes,
and its destination; with such other matters as the practice of the place
requires.
     2. This document is indispensably necessary in time of war for the
safety of every neutral vessel. Marsh. Ins. B. 1, c. 9, s. 6, p. 406, b.
     3. In most countries of continental Europe passports are given to
travellers; these are intended to protect them on their journey from all
molestation, while they are obedient to the laws. Passports are also granted
by the secretary of state to persons travelling abroad, certifying that they
are citizens of the United States. 9 Pet. 692. Vide 1 Kent, Com. 162, 182;
Merl. Repert. h.t.



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.