United Airlines, Inc. (commonly referred to as United) is a major U.S. airline headquartered at Willis Tower in Chicago, Illinois. United operates a large domestic and international route network spanning cities large and small across the United States and all six inhabited continents.

Measured by fleet size and the number of routes, it is the third-largest airline in the world.

United has eight hubs, with Chicago–O'Hare being its largest in terms of passengers carried and the number of departures. It is a founding member of the Star Alliance, the world's largest airline alliance with a total of 28 member airlines. Regional service is operated by independent carriers under the brand name United Express. The United brand name was established by the amalgamation of several airlines in the late 1920s, the oldest of these being Varney Air Lines, which was founded in 1926 but was taken over by Continental Airlines management in 2010 in what was described as a "merger of equals" and a newly created holding company for the airlines.