The U.S. Census Bureau does not define the Southeast.
Still, it does define the "East South Central" and "South Atlantic" regions.
The "East South Central" region includes Kentucky, Tennessee, Mississippi and Alabama. The "South Atlantic" includes Florida, Georgia, South Carolina, North Carolina, Virginia, West Virginia, Maryland, Delaware and Washington, D.C.
That means that the Census Bureau roughly endorses the division of Southeast and Southwest using the Mississippi River, but it uses the older, outmoded definition of the South using the Mason-Dixon line below Pennsylvania.
Other than Johnny Cash, few people consider Maryland, Delaware and D.C. to be Southern.
Retired UNC-Chapel Hill professor John Shelton Reed said the Census Bureau periodically considers cutting Maryland and Delaware out of its Southern classification, but it has so far ended up sticking with it simply because that's how it's always been done.
