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]

The Collaborative International Dictionary of English v.0.48:

Reprisal \Re*pris"al\ (r?-priz"al), n. [F. repr?saille, It.
   ripresaglia, rappresaglia, LL. reprensaliae, fr. L.
   reprehendere, reprehensum. See Reprehend, Reprise.]
   1. The act of taking from an enemy by way of reteliation or
      indemnity.
      [1913 Webster]

            Debatable ground, on which incursions and reprisals
            continued to take place.              --Macaulay.
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   2. Anything taken from an enemy in retaliation.
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   3. The act of retorting on an enemy by inflicting suffering
      or death on a prisoner taken from him, in retaliation for
      an act of inhumanity. --Vattel (Trans.)
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   4. Any act of retaliation. --Waterland.
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   Letters of marque and reprisal. See under Marque.
      [1913 Webster]