The Collaborative International Dictionary of English v.0.48:

Marque \Marque\, n. [F. marque, in lettre de marque letter of
   marque, a commission with which the commandant of every armed
   vessel was obliged to be provided, under penalty of being
   considered a pirate or corsair; marque here prob. meaning,
   border, boundary (the letter of marque being a permission to
   go beyond the border), and of German origin. See March
   border.] (Law)
   A license to pass the limits of a jurisdiction, or boundary
   of a country, for the purpose of making reprisals.
   [1913 Webster]

   Letters of marque, Letters of marque and reprisal, a
      license or extraordinary commission granted by a
      government to a private person to fit out a privateer or
      armed ship to cruise at sea and make prize of the enemy's
      ships and merchandise. The ship so commissioned is
      sometimes called a letter of marque.
      [1913 Webster]

WordNet (r) 3.0 (2006):

letters of marque
    n 1: a license to a private citizen to seize property of another
         nation [syn: letter of marque, letters of marque,
         letter of mark and reprisal]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "letters of marque":
   Pastoral Epistle, aerogram, air letter, air piracy, airgraph,
   airplane hijacking, billet-doux, buccaneering, bull, chain letter,
   dead letter, dimissorial, dimissory letter, drop letter,
   encyclical, fan letter, form letter, freebooting, letter of credit,
   letter of introduction, letters credential, letters of request,
   letters overt, letters patent, letters rogatory, love letter,
   monitory, newsletter, nixie, open letter, pastoral letter, piracy,
   poison-pen letter, privateering, round robin, skyjacking