What does "passport" mean?

Passport: A document carried by neutral merchant vessels in time of war, to certify their nationality and protect them from belligerents; a sea letter.

Additional senses

  1. 2.A license granted in time of war for the removal of persons and effects from a hostile country; a safe-conduct. Burrill.
  2. 3.Figuratively: Anything which secures advancement and general acceptance. Sir P. Sidney. His passport is his innocence and grace. Dryden.

Sources