Short answers, pulled from the story.
A capital ship is the most important type of warship in a navy, generally the largest and most powerful vessel in a fleet. There is no single formal criterion for the classification, but it is a strategic concept used to compare naval strength across theatres without focusing on specific tonnage or gun calibres.
The term capital ship was coined in 1909 and was formally defined in a series of international naval treaties in the 1920s and 1930s, including the 1922 Washington Naval Treaty, the 1930 London Naval Treaty, and the 1936 Second London Naval Treaty.
During the Age of Sail, capital ships were defined by the Royal Navy's rating system as ships of the first, second, third, and fourth rates. First-rate ships carried 100 or more guns; second-rates carried 90-98 guns; third-rates carried 64-80 guns; and fourth-rates carried 46-60 guns. By 1756, fourth-rates were considered too weak for the line of battle and were relegated to ancillary duties.
Aircraft carriers were universally recognised as capital ships by late 1942. Only full-size fleet carriers qualified; light carriers built on cruiser hulls and escort carriers built on merchant ship hulls were excluded. The shift followed the Japanese attack on Pearl Harbor in 1941, which forced the U.S. Navy to rely primarily on its carriers after eight Pacific-fleet battleships were sunk or damaged.
The United States operates 11 active duty supercarriers, each capable of carrying and launching nearly 100 tactical aircraft. It also possesses nine amphibious assault ships that, in their Sea Control Ship configuration, are equivalent to the light VSTOL carriers operated by other nations.
Many navies, including the Royal Navy and the United States Navy, consider nuclear submarines to be capital ships. However, nuclear submarines are primarily counted as part of a nation's nuclear deterrent force and do not share the sea control mission of traditional capital ships, making their inclusion a matter of ongoing debate.