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.
A document carried by neutral merchant vessels in time of war, to certify their nationality and protect them from belligerents; a sea letter.
A license granted in time of war for the removal of persons and effects from a hostile country; a safe-conduct.
Figuratively: Anything which secures advancement and general acceptance.