I think it is an unwritten rule mostly but, a major is an airline that does over $1billion in revenue, regardless of whether its outsourced flying (think Republic or Skywest) or not, like SWA.
A Legacy also meets the req's of a major but does its own flying and it's brand name has been around for 50+ years. So maybe in a few more years SWA will be considered a Legacy?
This is how it was explained to me anyways...