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A heavy cruiser was a type of cruiser, a naval warship designed for long range and high speed, armed generally with naval guns of roughly 203 mm (8 inches) in calibre, whose design parameters were dictated by the Washington Naval Treaty of 1922 and the London Naval Treaty of 1930. Heavy cruisers were generally larger, more heavily-armed and more heavily-armoured than light cruisers while being smaller, faster, and more lightly-armed and armoured than battlecruisers and battleships. Heavy cruisers were assigned a variety of roles ranging from commerce raiding to serving as 'cruiser-killers,' i.e. hunting and destroying similarly-sized ships.
The heavy cruiser is part of a lineage of ship design from 1915 through the early 1950s, although the term "heavy cruiser" only came into formal use in 1930. The heavy cruiser's immediate precursors were the light cruiser designs of the 1900s and 1910s, rather than the armoured cruisers of the years before 1905. When the armoured cruiser was supplanted by the battlecruiser, an intermediate ship type between this and the light cruiser was found to be needed—one larger and more powerful than the light cruisers of a potential enemy but not as large and expensive as the battlecruiser so as to be built in sufficient numbers to protect merchant ships and serve in a number of combat theatres.
With their intended targets being other cruisers and smaller vessels, the role of the heavy cruiser differed fundamentally from that of the armoured cruiser. Also, the heavy cruiser was designed to take advantage of advances in naval technology and design. Typically powered by oil-fired steam turbines rather than the reciprocating steam engines of the armoured cruiser, heavy cruisers were capable of far faster speeds and could cruise at high speed for much longer than could an armoured cruiser. They used uniform main guns, mounted in centre-line superfiring turrets rather than casemates. Casemate guns and a mixed battery were eliminated to make room for above deck torpedoes, and ever-increasing and more effective anti-aircraft armaments. They also benefited from the superior fire control of the 1920s and continually upgraded through the 1950s. Late in the development cycle radar and electronic countermeasures would also appear and rapidly gain in importance.