Chicago O'Hare International Airport (IATA: ORD, ICAO: KORD, FAA LID: ORD) is a major international airport serving Chicago, Illinois, located on the city's Northwest Side, approximately 17 miles (27 km) northwest of the Loop business district. Operated by the Chicago Department of Aviation and covering 7,627 acres (3,087 ha), O'Hare has non-stop flights to 214 destinations in North America, South America, the Caribbean, Europe, Africa, Asia, the Middle East, Oceania, and the North Atlantic region as of November 2022. As of 2023, O'Hare is considered the world's most connected airport. It is also U.S.A.'s 4th busiest airport, and 7th biggest airport.

Designed to be the successor to Chicago's Midway International Airport, itself once nicknamed the "busiest square mile in the world," O'Hare began as an airfield serving a Douglas manufacturing plant for C-54 military transports during World War II. It was renamed Orchard Field Airport in the mid-1940s and assigned the IATA code ORD. In 1949, it was renamed after aviator Edward "Butch" O'Hare, the U.S. Navy's first Medal of Honor recipient during that war. As the first major airport planned after World War II, O'Hare's innovative design pioneered concepts such as concourses, direct highway access to the terminal, jet bridges, and underground refueling systems.

O'Hare became famous during the jet age, holding the distinction as the world's busiest airport by passenger traffic from 1963 to 1998. It still ranks as one the busiest airports in the world, according to the Airports Council International rankings. In 2019, O'Hare had 919,704 aircraft movements, averaging 2,520 per day, the most of any airport in the world in part because of a large number of regional flights.