An airline is a company that provides air transport services for traveling passengers and/or freight. Airlines use aircraft to supply these services and may form partnerships or alliances with other airlines for codeshare agreements, in which they both offer and operate the same flight. Generally, airline companies are recognized with an air operating certificate or license issued by a governmental aviation body. Airlines may be scheduled or charter operators.

The first airline was the German airship company DELAG, founded on November 16, 1909. The four oldest non-airship airlines that still exist are the Netherlands' KLM (1919), Colombia's Avianca (1919), Australia's Qantas (1920) and the Czech Republic's Czech Airlines (1923).

Airline ownership has seen a shift from mostly personal ownership until the 1930s to government-ownership of major airlines from the 1940s to 1980s and back to large-scale privatization following the mid-1980s. Since the 1980s, there has also been a trend of major airline mergers and the formation of airline alliances. The largest alliances are Star Alliance, SkyTeam and Oneworld, and these three collectively account for more than 60% of global commercial air traffic in 2015. Airline alliances coordinate their passenger service programs (such as lounges and frequent-flyer programs), offer special interline tickets and often engage in extensive codesharing (sometimes systemwide).

As of 2019, the largest airline by passengers carried and fleet size was the American Airlines Group, while Delta Air Lines was the largest by revenue. Lufthansa Group was the largest by number of employees, FedEx Express by freight ton-kilometers, Turkish Airlines by number of countries served and UPS Airlines by number of destinations served (though United Airlines was the largest passenger airline by number of destinations served).