Sea letter, also known as a letter of marque, is a legal document authorizing a ship owner to attack enemy vessels. Synonyms for sea letter include letter of marque and reprisal, privateering permit, commission, and naval certificate. These documents were commonly used during times of war or conflict, allowing private ships to engage in attacks on enemy vessels without being considered pirates. Privateering was a lucrative business for ship owners, providing an alternative to military service for many sailors. However, the practice was eventually abolished by many nations due to ethical concerns and the rise of professional navies.