What is a merchant ship that is privately is privately owned and armed with weaponds called?

A merchant ship that is privately owned and armed with weapons is known as a "privateer." Privateers were authorized by a government during wartime to attack enemy ships and seize their cargo, operating under a letter of marque. They played a significant role in naval warfare, particularly from the 16th to the 18th centuries, allowing private individuals to engage in acts of war while sharing in the profits.