United Airlines, Inc. is a major airline in the United States headquartered in Chicago, Illinois that operates an extensive domestic and international route network across the United States and six continents with more destinations than any other airline. Regional service operated by independent carriers under the brand name United Express feeds its eight hubs and the Star Alliance, of which United was one of the five founding airlines, extends its network throughout the world.

United was formed beginning in the late 1920s as an amalgamation of several airlines, the oldest of these being Varney Air Lines,created in 1926 by Walter Varney who later co-founded the predecessor to Continental Airlines. Since Varney was a part of United, the founding year of United is 1926, making United the oldest commercial airline in the United States. United has ranked among the largest airlines in the world since its founding, often as a result of mergers and acquisitions.