During the Age of Sail, the term 'cruiser' was a general one, used to refer to refer to any ship whose primary role was to 'cruise' - to sail independently of the battlefleet. In this sense, it covered the whole spectrum of ships smaller than a ship-of-the-line; sloops, corvettes and frigates. The main dividing line was in terms of their armament. These cruisers had a small armament, typically under 50 guns, and these guns were also smaller than those carried by ships of the line. With such a light armament, they could not stand in the line of battle, and so they were relegated to cruising the sea lanes.

As we move through the 19th Century, technology changed things. Improvements in armament technology, especially the development of explosive shells, meant that smaller ships could actually pose a threat to larger ones. Armour swung the pendulum even further towards the smaller ships. Judging by their armament, the first ironclads, Gloire and Warrior, were classed as frigates; but in terms of their combat abilities, they were as capable as any older ship of the line. The advent of the monitor (and the later battleships inspired by it), which carried only a few guns, made a further mockery of the older system of ship classification. The advent of steam engines, meanwhile, allowed the larger ships to catch up with smaller ones. As a result, two new classifications began to emerge. Battleships were ships built or intended to sit in the line of battle, while cruisers were those ships intended to operate independently of it. These distinctions were officially introduced to the Royal Navy in the latter half of 1887.

The cruisers of the First and Second World Wars were, fundamentally, developments of these late-Victorian cruisers. They were built to serve in similar roles, and had a clear chain of ancestors stretching back to those ships. It made sense to keep calling them cruisers, rather than switching arbitrarily back to an obsolete designation. You do, however, see 'frigate' re-emerging as a classification during the Second World War. This was somewhat of an accident. During the First World War, the RN had introduced a new type of anti-submarine and minesweeping escort, which they classified as sloops. During the run-up to the Second, a smaller, cheaper 'sloop' was procured, and given the classification of corvette, becoming the 'Flower' class - this actually reversed the historical relationship between sloops and corvettes, but corvette was the only historical word for a small warship available that did not require re-assigning large swathes of existing ships. Then, in 1940-2, a larger, faster corvette was designed for the RN. These ships were, when designed, called 'twin-screw corvettes', having an extra engine and propeller compared to the 'Flower's, but this was clumsy. Against the Admiralty's disapproval, the Royal Canadian Navy began to refer to the ships as 'frigates', completing the trifecta of age-of-sail small warships. Ultimately, the name stuck.