An airline refers to an enterprise that transports people or goods by air with various means of air transportation. Airlines are enterprises that provide civil aviation services for passengers and goods by various air carriers. The aircraft used by airlines can be owned or leased, can provide services independently, and can also form partners or alliances with other airlines.

The scale of airlines can range from a single plane to an international airline with hundreds of planes providing various global services. The service scope of airlines can be divided into intercontinental, intercontinental and domestic, and can also be divided into flight service and charter service. Major airlines usually register call signs with ICAO. Call sign is usually the name of the airline, but there are exceptions. Add the flight number after the call sign, and some large airlines with full loads will add heavy (cathay-seven-one-seven-heavy) after the call sign to let the air traffic controller know that the flight is extremely heavy and it is impossible to execute some instructions (such as keeping a low speed and flying to a higher altitude).