Who needs the federal government? We've got football.
Though the U.S. Census Bureau does not define the Southeastern region in its reports, another major — more important? — agency does: The Southeastern Conference.
The college athletic conference headquartered in Alabama has its own roster of states it considers to be in the Southeast:
Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Mississippi, South Carolina and Tennessee.
It does not include North Carolina or Virginia, which are part of the Atlantic Coast Conference but are undoubtedly in the Southeast. It also skips West Virginia, a borderline case.
The definition is important because a recent political ad compares tax rates in the Southeast, which obviously differ depending on which states you include.