Throughout naval history, it was used for some of the largest and most common warships in a given fleet, but alternate designations like Star Cruiser was also applied, like in the navy of the Galactic Empire[2] and the Rebel Alliance.[3] Ships that were designated with other names were also often referred to as cruisers.[4][5]
In at least one classification system, cruisers could be divided into three categories: light, medium and heavy. The light cruisers were at the lower end of the cruiser interval, around 350–400 meters, which likewise were sometimes interchangable with frigates under the Anaxes War College System[8], medium cruisers between 400 and 500 meters, and heavy cruisers 600 meters and beyond. The latter group is also supplied under a completely separate class under the Anaxes War College System.[8] The alternate designation of battle cruiser was also used for some ships in the same size brackets.[9]
With a variety of different ship classifications in use, some of these used the "cruiser" definition for diminutive patrol ships like the Guardian-class customs vessel.[10][11]
In the real world, the term cruiser originates in the age of sail, when it denoted simply a ship assigned to cruise independently of large battlefleets, either as a scout operating ahead of the main fleet, or on detached duty. This of course defined cruisers as ships which did not fight in the line of battle, so that the term was particularly applied to frigates, fast, mid-sized vessels that were not designed to stand up to the firepower of ships of the line.
As the ironclad developed the later nineteenth century, the term "cruiser" came to be applied definitively to a class of ship designed for duties outside the battle line, initially serving in more distant waters as raiders or in patrol squadrons. These ships were essentially lighter-weight cousins of contemporary battleships, with less-heavy armor and main guns, and they were divided into two types: armored cruisers, which had armored belts on the sides of the hull in the manner of contemporary battleships, and protected cruisers, which simply had an armored deck over their vulnerable engines and magazines. Early ironclad cruisers were no faster or smaller than contemporary battleships, but as their speed improved, scouting for major battlefleets once again became an important role for the cruiser.
In the early twentieth century, inspired by the new generation of battleships represented by HMS Dreadnought, the armored cruiser developed into the battlecruiser, which carried guns of the same caliber as battleships, and came to be thought of as a type of fast, light-armored capital ship that could fight with the battlefleet. Actual cruiser construction shifted to smaller, faster "light armored cruisers", soon simply called light cruisers, with guns of six-inch and four-inch calibers. By the end of the First World War, the largest "light cruisers" had in turn grown in size to carry 7½-inch main guns.
The inter-war naval treaties formally defined cruisers by three main constraints: limiting their displacement to 10,000 tons; redefining a "light cruiser" with nothing more than six-inch guns; and formalizing a new term, heavy cruiser, for ships with guns of up to eight-inch caliber. These caliber-based definitions remained in use through World War Two, and after the decommissioning of the last battleships, cruisers became the largest traditional surface-combat warships in service, though in modern times, whether a ship is a "cruiser" or not is largely a matter of what she is officially defined as one. Russia uses the term for all large surface warships, including the massive Kirov-class "battlecruisers" and their aircraft carriers, a usage which by 1975 had led to a perception of a "cruiser gap" in the United States Navy, which had only one modern warship designated as a cruiser; as a result, the "cruiser" designation was revived in US service, although modern USN "cruisers" have, without exception, been designed as large destroyers. The Royal Navy has, in contrast, designed ships as cruisers which entered service under other designations: the Type 82 destroyer, and the Invincible-class aircraft carrier.
In Star Wars, the standard definition of the "cruiser" type is probably closest to the modern Russian usage, although under the Empire, the separation of most large cruisers into a sub-category as Star Destroyers largely reduces the "pure" cruiser to a secondary role more akin to historical usage. Something similar is true in alternative designation systems that add distinct battleship categories, although here the size definition of "cruiser" can be different.