If I recall... as long as you post more than $1 Billion in revenue, you are considered a "Major" by the DOT.
That doesn't mean those "Majors" earn $1 billion in profit...
As a result, many of the "Regional" airlines are considered "Major" airlines by the DOT... such as ExpressJet, SkyWest, Envoy, etc.
That said... in the traditional sense... we usually define the airlines as:
- Regional: Any airline that doesn't sell their own tickets and sub-contracts flying from the major/legacy carriers
- Major: Any airline that sells their own tickets, synonymous with "mainline" carrier. AA, JetBlue, Frontier, Spirit, Virgin America, United, etc.
- Legacy: An interstate airline that was established prior to deregulation in the 70's... American, Delta, and United are all that's left. TWA, PanAm, Eastern, Braniff, etc. are example of defunct "Legacy" carriers.