HMS Ark Royal (pennant number 91) was an aircraft carrier of the Royal Navy that was operated during the Second World War.

Designed in 1934 to fit the restrictions of the Washington Naval Treaty, Ark Royal was built by Cammell Laird at Birkenhead, England, and completed in November 1938. Her design differed from previous aircraft carriers. Ark Royal was the first ship on which the hangars and flight deck were an integral part of the hull, instead of an add-on or part of the superstructure. Designed to carry a large number of aircraft, she had two hangar deck levels. She was used during a period that first saw the extensive use of naval air power; several carrier tactics were developed and refined aboard Ark Royal.

Ark Royal operated in some of the most active naval theatres of the Second World War. She was involved in the first aerial U-boat kills of the war, operations off Norway, the search for the German battleship Bismarck, and the Malta Convoys. Ark Royal survived several near misses and gained a reputation as a 'lucky ship'. She was torpedoed on 13 November 1941 by the German submarine U-81 and sank the following day. One of her 1,488 crew members was killed. Her sinking was the subject of several inquiries, with investigators keen to know how the carrier was lost in spite of efforts to save the ship and tow her to the naval base at Gibraltar. They found that several design flaws contributed to the loss, which were rectified in new British carriers.

The wreck was discovered in December 2002 by an American underwater survey company using sonar mounted on an autonomous underwater vehicle, under contract from the BBC for the filming of a documentary about the ship, at a depth of about 3,300 feet (1,000 m) and approximately 30 nautical miles (56 km; 35 mi) from Gibraltar.

An aircraft carrier is a warship that serves as a seagoing airbase, equipped with a full-length flight deck and facilities for carrying, arming, deploying, and recovering aircraft. Typically, it is the capital ship of a fleet, as it allows a naval force to project air power worldwide without depending on local bases for staging aircraft operations. Carriers have evolved since their inception in the early twentieth century from wooden vessels used to deploy balloons to nuclear-powered warships that carry numerous fighters, strike aircraft, helicopters, and other types of aircraft. While heavier aircraft such as fixed-wing gunships and bombers have been launched from aircraft carriers, one has yet to successfully land on one. By its diplomatic and tactical power, its mobility, its autonomy and the variety of its means, the aircraft carrier is often the centerpiece of modern combat fleets. Tactically or even strategically, it replaced the battleship in the role of flagship of a fleet. One of its great advantages is that, by sailing in international waters, it does not interfere with any territorial sovereignty and thus obviates the need for overflight authorizations from third-party countries, reduces the times and transit distances of aircraft and therefore significantly increase the time of availability on the combat zone.

There is no single definition of an "aircraft carrier", and modern navies use several variants of the type. These variants are sometimes categorized as sub-types of aircraft carriers, and sometimes as distinct types of naval aviation-capable ships. Aircraft carriers may be classified according to the type of aircraft they carry and their operational assignments. Admiral Sir Mark Stanhope, RN, former First Sea Lord (head) of the Royal Navy, has said, "To put it simply, countries that aspire to strategic international influence have aircraft carriers." Henry Kissinger, while United States Secretary of State, also said: "An aircraft carrier is 100,000 tons of diplomacy."

As of March 2022, there are 47 active aircraft carriers in the world operated by fourteen navies. The United States Navy has 11 large nuclear-powered fleet carriers—carrying around 80 fighters each—the largest carriers in the world; the total combined deck space is over twice that of all other nations combined. As well as the aircraft carrier fleet, the US Navy has nine amphibious assault ships used primarily for helicopters, although these also each carry up to 20 vertical or short take-off and landing (V/STOL) fighter jets and are similar in size to medium-sized fleet carriers. The United Kingdom and China each operate two aircraft carriers. France, India, and Russia each operate a single aircraft carrier with a capacity of 30 to 60 fighter jets. Italy operates two light fleet carriers and Spain operates one. Helicopter carriers are operated by Japan (4, two of which are being converted to operate V/STOL fighters), France (3), Australia (2), Egypt (2), South Korea (2), China (2), Thailand (1) and Brazil (1). Future aircraft carriers are under construction or in planning by Brazil, China, France, India, Russia, South Korea, Turkey and the US.