Long ago, a privately-owned ship used during wartime was known as a privateer. Often the captain and crew of such a ship were also called privateers.
The term privateer is sometimes used interchangeably with "pirate," and in fact these independent sailors frequently acted as pirates, looting and seizing other seagoing vessels for their own benefit. Originally, the term designated ships that were commissioned during wartime by a government, so they could aid in fighting the enemy at sea. The word comes from the phrase "private man of war," and is modeled after "volunteer" and "buccaneer."