Chicago O'Hare International Airport (IATA: ORD, ICAO: KORD, FAA LID: ORD), also known as O'Hare Airport, O'Hare Field, Chicago International Airport, or simply O'Hare, is a major airport located in the northwestern-most corner of Chicago, Illinois, United States, 17 miles (27 km) northwest of the Chicago Loop (the CBD). It is the primary airport serving the Chicago area, with Chicago Midway International Airport, about 10 miles (16 km) closer to the Loop, serving as a secondary airport for low-cost carriers.

United Airlines (including United Express) is the largest airline at O'Hare, carrying over 45% of passengers. O'Hare is the second-largest hub for United Airlines after Houston-Bush. American Airlines (including American Eagle) has the second largest operation at O'Hare, carrying 37.08% of passengers. O'Hare is American Airlines' second-largest hub after Dallas/Fort Worth.[4]

Until 2005, O'Hare was the world's busiest airport in regards to takeoffs and landings. Up till 1998, O'Hare was the busiest airport in the world in terms of the number of passengers. Mainly due to limits imposed by the federal government to reduce flight delays at O'Hare,[5] Hartsfield-Jackson Atlanta International Airport became the busiest by the former metric in 2005. O'Hare is the world's second busiest airport in terms of aircraft movements. In 2008, the airport had 881,566 aircraft operations, an average of 2,409 per day (64% scheduled commercial, 33% air taxi, 3% general aviation and <1% military).[1] O'Hare is the fifth busiest airport in the world (after Atlanta, Beijing, London Heathrow and Tokyo Haneda) with 66,633,503 passengers passing through the airport in 2012, a -0.1% change from 2011.[3] O'Hare has a strong international presence, with flights to more than 60 foreign destinations: it is the fourth busiest international gateway in the United States behind John F. Kennedy International Airport in New York City, Los Angeles International Airport and Miami International Airport.

O'Hare has been voted the "Best Airport in North America" for 10 years by two separate sources: Readers of the U.S. Edition of Business Traveler Magazine (1998–2003) and Global Traveler Magazine (2004–2007).[6] Travel and Leisure magazine's 2009 "America's Favorite Cities" ranked Chicago's Airport System (O'Hare and Midway) the second-worst for delays, New York City's airport system (JFK, Newark Liberty, and LaGuardia) being the first.[7] O'Hare currently accounts for over a sixth of the nation's total flight cancellations.[8]