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]