An aircraft catapult is a device used to help fixed-wing aircraft reach liftoff speed (V_LOF) faster during takeoff, typically when trying to take off from a very short runway, as otherwise the aircraft engines alone cannot accelerate to sufficient airspeed quickly enough for the wings to generate the lift needed to sustain flight. Launching via catapults enables aircraft that typically are only capable of conventional takeoffs, especially heavier aircraft with significant payloads, to perform short takeoffs from the roll distances of light aircraft. Catapults are usually used on the deck of a ship — such as the flight deck of an aircraft carrier — as a form of assisted takeoff for navalised aircraft, but can also be installed on land-based runways, although this is rare.

The catapult system used on aircraft carriers consists of a straight track or slot built into the flight deck, on top of which is a sliding piece called a shuttle, which protrudes above the deck and is hooked onto the nose gear of the aircraft; or in some cases a wire rope called a catapult bridle is attached between the aircraft and the catapult shuttle. When launching, the shuttle is driven forward by force mechanisms within the track system, "hurling out" the aircraft and imparting an additional propulsion on top of the aircraft's own engine thrust to help it accelerate. Other forms of catapult have been used historically, such as mounting a launching cart holding a seaplane on a long girder-built structure mounted on the deck of a warship or merchant ship, but most catapult systems share a similar sliding track concept.

Historically it was most common for seaplanes (which have pontoons instead of landing gears and thus cannot utilize runways) to be catapulted from ships onto nearby water for takeoff, allowing them to conduct aerial reconnaissance missions and be crane-hoisted back on board during retrieval, although by the late First World War their roles are largely supplanted by the more versatile biplanes that can take off and land on carrier decks unassisted. During the Second World War before the advent of escort carriers, monoplane fighter aircraft (notably the Hawker Hurricane) would sometimes be catapulted from "catapult-equipped merchant" (CAM) vessels for one-way sorties to repel enemy aircraft harassing shipping lanes, forcing the returning pilot to either divert to a land-based airstrip, jump out by parachute, or ditch in the water near the convoy and wait for rescue. By the time fleet carriers became the norm in WW2, catapult launches have become largely unnecessary and carrier-based fighter-bombers would routinely perform self-powered takeoffs and landings off and onto carrier decks, especially during the Pacific War between the United States and the Empire of Japan. However, escalating arms races during the Cold War accelerated the adoption of the heavier jet aircraft for naval operations, thus motivating the development of new catapult systems, especially after the popularization of angled flight decks further limited the practical distance available as takeoff runways. Nowadays, jet aircraft can launch from aircraft carriers via either catapults or ski-jump deck, and perform optics-assisted landing onto the same ship with help from decelerative arresting gears.

Different means have been used to propel the catapult, such as weight and derrick, gunpowder, flywheel, compressed air, hydraulic, steam power, and solid fuel rocket boosters. Since the second half of the 20th century, steam catapults have been the mainstay form of aircraft carrier catapults, especially on the nuclear-powered Nimitz-class supercarriers, and is the core component of the catapult-assisted takeoff but arrested recovery (CATOBAR) system. Into the 21st century, General Atomics has developed an alternating current linear motor-based electromagnetic catapult system called the Electromagnetic Aircraft Launch System (EMALS), which is installed on the United States Navy's new Gerald R. Ford-class aircraft carriers. A direct current-based electromagnetic catapult has also been developed by the People's Republic of China for the People's Liberation Army Navy's Type 003 aircraft carrier Fujian, as well as the upcoming Type 076 amphibious assault ship Sichuan (currently fitting out) and Type 004 aircraft carriers (in-construction).

Aviation pioneer and Smithsonian Secretary Samuel Langley used a spring-operated catapult to launch his successful flying models and his failed Aerodrome of 1903. Likewise the Wright Brothers beginning in 1904 used a weight and derrick styled catapult to assist their early aircraft with a takeoff in a limited distance.

On 31 July 1912, Theodore Gordon Ellyson became the first person to be launched from a U.S. Navy catapult system. The Navy had been perfecting a compressed-air catapult system and mounted it on the Santee Dock in Annapolis, Maryland. The first attempt nearly killed Lieutenant Ellyson when the plane left the ramp with its nose pointing upward and it caught a crosswind, pushing the plane into the water. Ellyson was able to escape from the wreckage unhurt. On 12 November 1912, Lt. Ellyson made history as the Navy's first successful catapult launch, from a stationary coal barge. On 5 November 1915, Lieutenant Commander Henry C. Mustin made the first catapult launch from a ship underway.

The US Navy experimented with other power sources and models, including catapults that utilized gunpowder and flywheel variations. On 14 December 1924, a Martin MO-1 observation plane flown by Lt. L. C. Hayden was launched from USS Langley using a catapult powered by gunpowder. Following this launch, this method was used aboard both cruisers and battleships.

By 1929, the German ocean liners SS Bremen and Europa had been fitted with compressed-air catapults designed by the Heinkel aviation firm of Rostock, with further work with catapult air mail across the South Atlantic Ocean, being undertaken during the first half of the 1930s, with Dornier Wal twin-engined flying boats.