Airlines are classified in several different ways, however, the U.S. Department of Transportation, the primary government entity that oversees national transportation policy defines airlines based strictly on annual revenues as follows:

Major Airline:
A major airline is one that generates over $1 billion in annual operating revenues. This list currently includes Alaska Airlines, American Airlines, American Eagle, ATA Holdings, America West, Continental Air Lines, Delta Air Lines, DHL Airways, FedEx, Northwest Airlines, Southwest Airlines, United, United Parcel Service (UPS), and US Airways.

National Airline:
A national airline is one that generates between $100 million and $1 billion in annual operating revenues. The national airlines tend to serve particular regions of the country; however, some do fly long-haul flights. Some examples are: AirTran, Frontier Airlines, JetBlue, and Midwest Express.

Regional Airline:
A regional airline is one that generates under $100 million in revenues and generally serves small communities. The Regional Airline Association defines regional airlines as "...operat(ing) short and medium haul scheduled airline service connecting smaller communities with larger cities and connecting hubs. The airlines' fleet primarily consists of 19 to 68 seat turboprops and 30 to 100 seat regional jets." Some examples are: American Eagle Airlines, Atlantic Southeast Airlines, Atlantic Coast Airlines, and SkyWest Airlines.