Southern cooking is regional cuisine to Americans of other regions, as is Tex Mex or Heartland, etc. The defining area historically was NYC and Boston. Even Baltimore and Tidewater, outer Long Island etc were regional. As communication, travel, and transportation of goods became faster and more reliable, seaonality and regionality waned from our culinary consciousness. We all were making Hamburger Helper, lol.
We probably misuse the word ethnic. But when ingredients and techniques come from a different country and/or a different culture outside our own, we refer to it as ethnic. I think the fact that American cuisine has moved beyond its English/German heritage and embraced so many other elements, fusion or some other similar bonding, some of these distinctions become difficult to pin down.
Interesting things can happen. I served beer braised grilled brats with onions and fine mustard to a group of friends from Pennsylvania, (of German origin) who had never had other than a hotdog before. To me a brat is just another sausage, one of many available. To these guys this was foreign imported food from...WISCONSIN or EUROPE or is this CANADIAN FOOD??? So it really depends on one's own perspective as well.
Well I'm glad regional cuisine, seasonal cuisine, and ethnic cuisine, along with fusion are all part of our culinary consciousness again.
Great topic!