What is Privateer?

Legal Definition
A privateer was a private person or ship that engaged in maritime warfare under a commission of war. The commission, also known as a letter of marque, empowered the person to carry on all forms of hostility permissible at sea by the usages of war, including attacking foreign vessels during wartime and taking them as prizes. Captured ships were subject to condemnation and sale under prize law, with the proceeds divided between the privateer sponsors, shipowners, captains and crew. A percentage share usually went to the issuer of the commission. Since robbery under arms was common to seaborne trade, all merchant ships were already armed. During war, naval resources were auxiliary to operations on land so privateering was a way of subsidizing state power by mobilizing armed ships and sailors.
-- Wikipedia
Legal Definition
War. A vessel owned by one or by a society of private individuals, armed and equipped at his or their expense, for the purpose of carrying on a maritime war, by the authority of one of the belligerent parties.

2. For the purpose of encouraging the owners of private armed vessels, they are usually allowed to appropriate to themselves the property they capture, or, at least, a large proportion of it. 1 Kent, Com. 96; Posh. du Dr. de Propr. n. 90 et seq. See 2 Dall. 36; 3 Dall. 334; 4 Cranch, 2; 1 Wheat. 46; 3 Wheat. 546; 2 Gall. R. 19; Id. 526; 1 Mason, R. 365 3 Wash. C. C. R. 209 2 Gall. R. 56; 5 Wheat. 338; Mann. Com. 1.16.
-- Bouviers Law Dictionary
Legal Definition
A vessel owned, equipped, and armed by one or more private individuals, and duly commissioned by a belligerent power to go on cruises and make war upon the enemy, usually by preying on his commerce.
-- Black's Law Dictionary
Legal Definition
A privately owned ship acting under letters of marque and reprisal. See Letters of marque and reprisal.
-- Ballentine's Law Dictionary