A light cruiser is a type of small or medium-sized warship. The term is a shortening of the phrase "light armored cruiser", describing a small ship that carried armor in the same way as an armored cruiser: a protective belt and deck. Prior to this smaller cruisers had been of the protected cruiser model, possessing armored decks only. While lighter and smaller than other contemporary ships they were still true cruisers, retaining the extended radius of action and self-sufficiency to act independently around the world. Through their history they served in a variety of roles, primarily as convoy escorts and destroyer command ships, but also as scouts and fleet support vessels for battle fleets.
The first small steam-powered cruisers were built for the British Royal Navy with HMS Mercury launched in 1878. Such second and third class protected cruisers evolved, gradually becoming faster, better armed and better protected. Germany took a lead in small cruiser design in the 1890s, building a class of fast cruisers—the —copied by other nations. Such vessels were powered by coal-fired boilers and reciprocating steam engines and relied in part on the arrangement of coal bunkers for their protection. The adoption of oil-fired water-tube boilers and steam turbine engines meant that older small cruisers rapidly became obsolete. Furthermore, new construction could not rely on the protection of coal bunkers and would therefore have to adopt some form of side armoring. The British Chatham group of cruisers were a departure from previous designs; with turbine propulsion, mixed coal and oil firing and a 2-inch protective armored belt as well as deck. Thus, by definition, they were armored cruisers, despite displacing only 4,800 tons; the light armored cruiser had arrived. The first true modern light cruisers were the which had all oil-firing and used lightweight destroyer-type machinery to make .
By World War I, British light cruisers often had either two and perhaps eight guns, or a uniform armament of 6-inch guns on a ship of around 5,000 tons, while German light cruisers progressed during the war from to guns.
