The capital ships of a navy are its "important" warships; the ones with the heaviest firepower and armor. 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 theater of operations without having to get bogged down in the details of tonnage and gun diameters.
In the 20th century, typical capital ships would be battleships, heavy cruisers, and aircraft carriers.
The definition of "capital ship" was formalized in the limitation treaties of the 1920s and 30s; see London Naval Treaty and Second London Naval Treaty.
