The term 'An American Airline' can be summarized by the crossword solution of 'UNITED'. UNITED Airlines is an American airline headquartered in Chicago, Illinois. It is the world's third-largest airline when measured by revenue, and the largest airline when measured by number of destinations served. UNITED operates a large domestic and international route network, with an extensive presence in the Asia-Pacific region. The airline is a founding member of the Star Alliance, the world's largest airline alliance.