A cruiser is a type of warship. The term has been in use for several hundred years, and has had different meanings throughout this period. During the Age of Sail, the term cruising referred to certain kinds of missions – independent scouting, commerce protection, or raiding – fulfilled by a frigate or sloop-of-war, which were the cruising warships of a fleet. Modern cruisers are generally the largest ships in a fleet after aircraft carriers, and can usually perform several roles.
In the middle of the 19th century, cruiser came to be a classification for the ships intended for cruising distant waters, commerce raiding, and scouting for the battle fleet. Cruisers came in a wide variety of sizes, from the medium-sized protected cruiser to large armored cruisers that were nearly as big (although not as powerful or as well-armored) as a pre-dreadnought battleship. With the advent of the dreadnought battleship before World War I, the armored cruiser evolved into a vessel of similar scale known as the battlecruiser. The very large battlecruisers of the World War I era that succeeded armored cruisers were now classified, along with dreadnought battleships, as capital ships.
By the early 20th century after World War I, the direct successors to protected cruisers could be placed on a consistent scale of warship size, smaller than a battleship but larger than a destroyer. In 1922, the Washington Naval Treaty placed a formal limit on these cruisers, which were defined as warships of up to 10,000 tons displacement carrying guns no larger than 8 inches in calibre; heavy cruisers had 8-inch guns, while those with guns of 6.1 inches or less were light cruisers, which shaped cruiser design until the end of World War II. Some variations on the Treaty cruiser design included the German Deutschland-class "pocket battleships" which had heavier armament at the expense of speed compared to standard heavy cruisers, and the American Alaska class, which was a scaled-up heavy cruiser design designated as a "cruiser-killer".
In the later 20th century, the obsolescence of the battleship left the cruiser as the largest and most powerful surface combatant after the aircraft carrier. The role of the cruiser varied according to ship and navy, often including air defense and shore bombardment. During the Cold War, the Soviet Navy's cruisers had heavy anti-ship missile armament designed to sink NATO carrier task forces via saturation attack. The U.S. Navy built guided-missile cruisers upon destroyer-style hulls (some called "destroyer leaders" or "frigates" prior to the 1975 reclassification) primarily designed to provide air defense while often adding anti-submarine capabilities, being larger and having longer-range surface-to-air missiles (SAMs) than early Charles F. Adams guided-missile destroyers tasked with the short-range air defense role. By the end of the Cold War, the line between cruisers and destroyers had blurred, with the Ticonderoga-class cruiser using the hull of the Spruance-class destroyer but receiving the cruiser designation due to their enhanced mission and combat systems. Indeed, the newest U.S. and Chinese destroyers (for instance the Zumwalt class and Type 055) are more heavily armed than some of the cruisers that they succeeded.
Currently only two nations operate cruisers: the United States and Russia. (BAP Almirante Grau was still in service with the Peruvian Navy until 2017, and was the last gun cruiser in service in any navy.)[en]