Legacy: Existed before deregulation AND did interstate flying (that last excludes SWA since they apparently only flew in TX before dereg). DL, UA, AA, AS, HA.

LCC: Vague and not clearly defined, but generally includes mid-range majors like JB. May include SWA and even AS, depends on who you ask.

ULCC: Vague and not clearly defined, but generally includes budget majors like Frontier, Allegiant, Spirit.

Major: All of the above. Sells their own tickets and operates at least narrowbody aircraft. The federal government has a different definition based on gross revenue, but we in the industry always bin airlines by their business model, not revenue.

Regional: Generally operates regional jets on contract with one of the legacy airlines. May do some or even all of their own branded flying. May do EAS. Operate about 50% of US domestic departures but most passenger have never heard of them.

Commuter: Typically operates smaller prop planes (even piston) on short routes. Typically own brand but may codeshare with a major.