The Dictionary of Cultural Literary describes the "Deep South" as the southernmost tier of southern states: South Carolina, Georgia, Florida, Alabama, Mississippi and Louisiana.
The National Endowment for the Humanities defines the Deep South as Alabama, Arkansas, Louisiana, Mississippi and Tennessee.
Princeton's WordNet describes it as South Carolina, Georgia, Alabama, Mississippi and Louisiana.
The "Deep South" is usually defined in opposition to the border states, or the Upper South states of Virginia, North Carolina, Tennessee, and Arkansas.