A letter of marque and reprisal (French: lettre de marque; lettre de course) was a government license in the Age of Sail that authorized a private person, known as a privateer or corsair, to attack and capture vessels of a nation at war with the issuer. Once captured, the privateer could then bring the case of that prize before their own admiralty court for condemnation and transfer of ownership to the privateer. A letter of marque and reprisal would include permission to cross an international border to effect a reprisal (take some action against an attack or injury) and was authorized by an issuing jurisdiction to conduct reprisal operations outside its borders.
Popular among Europeans from the late Middle Ages up to the 19th century, cruising for enemy prizes with a letter of marque was considered an honorable calling that combined patriotism and profit. Such privateering contrasted with attacks and captures of random ships, which was unlicensed and known as piracy; piracy was almost universally reviled. In reality, the differences between privateers and pirates were often at best subtle, and at worst more a matter of interpretation.
Source: Letter of marque – Wikipedia