The list of units
Privateers are small frigates that carry no nationality markings, allowing them to attack and be attacked without revealing the nationality of the ship. Thus, you can attack another civilization's shipping without precipitating war.
A coastal city needs both iron and saltpeter in its Strategic Resource box to build privateers.
Privateers were men who owned their own ship and were commissioned by a nation to attack the naval traffic of their foes. To facilitate anonymity, very little compensation was paid to the crew and captain by their patron government. Rather, they were given a portion of the cargo and goods seized in the execution of their duties. While this made the profession popular with their crew, it eventually became a critical element of the practices disbanding. When nations attempted to staff their own navies for brewing conflicts, they frequently were unable to secure the manpower needed (all able bodied sailors already in privateering). Privateers were widespread throughout most European history, but they finally vanished during the 19th century.