A privateer is a man or a ship serving a governmental organization in a paramilitary role, or a member of its crew. Often what was basically legitimized pirates, a privateer captain would operate under a government contract known as a Letter of Marque and Reprisal, giving the government a share of the profits in return for issuing it (usually one-fifth of the value of the captured ship and cargo). This contract gave him or her the authority to harass, seize and destroy the shipping of another organization, typically one that was considered an enemy in times of war.
In the Galactic Civil War, the Rebel Alliance would hire a number of privateers to harass Imperial shippings, a policy supported particularly by Ral'Rai Muvunc of Ordnance and Supply. The Empire themselves were known to use privateers on occasion.
Privateers were also common in the Hapes Cluster, where they seem to have served the interests of political factions in the Hapes Consortium. Prince Isolder Fairchild famously went undercover with the crew of Corellian pirate Grov Bricker to track down Harravan, the Hapan privateer who killed his brother, Kalen.