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The capital ships of a navy are its "important" warships; the ones with the heaviest firepower and armour. There is usually no formal criterion for the classification, but it is a useful concept when thinking about strategy, for instance to compare relative naval strengths in a theatre of operations without having to get bogged down in the details of tonnage and gun diameters. A capital ship is generally a leading or a primary ship in a fleet.

In the 20th century, especially in World Wars I and II, typical capital ships would be battleships, battlecruisers, and in WWII, aircraft carriers (though it took until late 1942 for carriers to be universally considered capital ships). All of the above ships were close to 20,000 tons displacement or heavier. Heavy cruisers, despite being important ships, were not considered capital ships.

An exception to the above in World War II was the Deutschland-class cruiser. Though this class was technically similar to a heavy cruiser, albeit with considerably heavier guns, they were generally regarded as capital ships (hence the British label "Pocket battleship"). The Alaska-class cruisers, despite being "supersized" heavy cruisers and not battleships/battlecruisers, were also considered by some to be capital ships.

During the Cold War, a Soviet Kirov-class large missile cruiser had a displacement great enough to rival WWII-era capital ships, perhaps defining a new battlecruiser for that era.

In the 21st century, the aircraft carrier is the last remaining capital ship, with firepower defined in decks available and aircraft per deck, rather than in guns and calibres. The United States has undeniable supremacy in both categories of aircraft carriers, possessing not only 11 active duty supercarriers each capable of carrying and launching nearly 100 tactical aircraft, but an additional 12 amphibious assault ships every bit as capable (in the "Sea Control Ship" configuration) as the light VSTOL carriers of other nations.

Ballistic missile submarines (or "boomers"), while important ships and in tonnage are similar to early battleships, are usually counted as part of a nation's nuclear deterrent force and do not share the sea control mission of traditional capital ships. (Although in some navies (Royal Navy and United States Navy), ballistic submarines are given names typically formerly given to battleships).

The definition of "capital ship" was formalized in the limitation treaties of the 1920s and 30s; see Washington Naval Treaty, London Naval Treaty, and Second London Naval Treaty.

Before the advent of the all-steel navy in the late 19th century, a capital ship was a warship of the First, Second or Third rates:

• 1st Rate: 100 or more guns, typically carried on three or four decks. Four-deckers tended to have problems with the waterline and the lowest deck could seldom fire except on the calmest of seas.
• 2nd Rate: 90–98 guns
• 3rd Rate: 64 to 80 guns (although 64-gun third-raters were very small and not very numerous in any era).

Frigates were ships of the fourth or fifth rate; a corvette was a ship of the sixth rate.

The term has also been adopted into science fiction literature and culture to describe large spaceships used in military contexts, particularly where other naval terms have also been adopted in similar fashion.

• Ship of the line