letter of marque

noun2

1. •
   (historical) An official commission from a government authorizing someone to seize the property or people of another nation; especially to attack merchant shipping of another nation.

   • A letter of marque came from the King / to the scummiest vessel I'd ever seen / God damn them all! I was told / We'd cruise the seas for American gold / We'd fire no guns, shed no tears. — 1976
2. •
   (historical) A ship that has been armed for such a task.