An aircraft carrier is a warship designed with a primary mission of deploying and recovering aircraft, acting as a seagoing airbase. Aircraft carriers thus allow a naval force to project air power great distances without having to depend on local bases for staging aircraft operations. They have evolved from wooden vessels used to deploy a balloon into nuclear powered warships that carry dozens of fixed and rotary wing aircraft.

Balloon carriers were the first ships to deploy manned aircraft, used during the 19th and early 20th century, mainly for observation purposes. The 1903 advent of fixed wing airplanes was followed in 1910 by the first flight of such an aircraft from the deck of a US Navy cruiser. Seaplanes and seaplane tender support ships, such as HMS  Engadine , followed. The development of flat top vessels produced the first large fleet ships. This evolution was well underway by the mid 1920s, resulting in ships such as the HMS  Hermes , Hōshō , and the Lexington -class aircraft carriers.

World War II saw the first large-scale use and further refinement of the aircraft carrier, spawning several types. Escort aircraft carriers, such as USS  Bogue , were built only during World War II. Although some were purpose-built, most were converted from merchant ships as a stop-gap measure to provide air support for convoys and amphibious invasions. Light aircraft carriers, such as USS  Independence , represented a larger, more "militarized" version of the escort carrier concept. Although the light carriers usually carried the same size air groups as escort carriers, they had the advantage of higher speed as they had been converted from cruisers under construction.

Wartime emergencies also saw the creation or conversion of unconventional aircraft carriers. CAM ships, like the SS  Michael E , were cargo-carrying merchant ships which could launch but not retrieve fighter aircraft from a catapult. These vessels were an emergency measure during World War II as were Merchant aircraft carriers (MACs), such as MV  Empire MacAlpine , another emergency measure which saw cargo-carrying merchant ships equipped with flight decks. Battlecarriers were created by the Imperial Japanese Navy to partially compensate for the loss of carrier strength at Midway. Two of them were made from Ise -class battleships during late 1943. The aft turrets were removed and replaced with a hangar, deck and catapult. The heavy cruiser Mogami concurrently received a similar conversion. This "half and half" design was an unsuccessful compromise, being neither one thing nor the other. Submarine aircraft carriers, such as the French Surcouf and the Japanese I-400 class submarine, which was capable of carrying 3 Aichi M6A Seiran aircraft, were first built in the 1920s, but were generally unsuccessful at war. Modern navies that operate such ships treat aircraft carriers as the capital ship of the fleet, a role previously played by the battleship. The change, part of the growth of air power as a significant factor in warfare, took place during World War II. This change was driven by the superior range, flexibility and effectiveness of carrier-launched aircraft.

Following the war, carrier operations continued to increase in size and importance. Supercarriers, typically displacing 75,000 tonnes or greater, have become the pinnacle of carrier development. Most are powered by nuclear reactors and form the core of a fleet designed to operate far from home. Amphibious assault ships, such as USS  Tarawa and HMS  Ocean , serve the purpose of carrying and landing Marines, and operate a large contingent of helicopters for that purpose. Also known as "commando carriers" or "helicopter carriers", many have a secondary capability to operate VSTOL aircraft.

Lacking the firepower of other warships, carriers by themselves are considered vulnerable to attack by other ships, aircraft, submarines, or missiles, and therefore travel as part of a carrier battle group (CVBG) for their protection. Unlike other types of capital ships in the 20th century, aircraft carrier designs since World War II have been effectively unlimited by any consideration save budgetary, and the ships have increased in size to handle the larger aircraft. The large, modern Nimitz  class of United States Navy carriers has a displacement nearly four times that of the World War II-era USS  Enterprise , yet its complement of aircraft is roughly the same—a consequence of the steadily increasing size of military aircraft over the years.