Chicago O'Hare International Airport (IATA: ORD, ICAO: KORD, FAA LID: ORD), also known as O'Hare Airport, O'Hare Field, Chicago Airport, 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. 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 domestic flights.

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 its second-largest hub after Dallas/Fort Worth.[3]

Prior to 2005, O'Hare was the world's busiest airport in terms of takeoffs and landings. Mainly due to limits imposed by the federal government to reduce flight delays at O'Hare,[4] Hartsfield-Jackson Atlanta International Airport became the busiest by that metric in 2005. O'Hare is now the third busiest airport in the world with 66,665,390 passengers passing through the airport in 2010, a +3.3% change from 2009.[5] It 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 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]

It is operated by the City of Chicago Department of Aviation, associated with an umbrella regional authority. Most of O'Hare Airport is in Cook County, but a section of the southwest part of the airport is in DuPage County. The Cook County portion is located within a section of the city of Chicago contiguously connected to the rest of the city via a narrow strip of land about 200 feet (61 m) wide, running along Foster Ave. from the Des Plaines River to the airport. This land was annexed into the city limits in the 1950s to assure the massive tax revenue associated with the airport being part of the city. The strip is bounded on the north by Rosemont and the south by Schiller Park.[9]