Not just the South, but more specifically, states that joined the Confederacy and, today, are most heavily influenced by Southern culture. Those are, from west to east, Texas, Arkansas, Louisiana, Mississippi, Alabama, Georgia, the Carolinas, Virginia, and Tennessee. Historically Florida, but most of the other states of Dixie would not be comfortable including Florida in their region anymore. Kentucky and Missouri were both strong Union states (65-75% in each) and today are essentially in the Midwest; they were never in Dixie in the first place.
Dixie also refers to the name of a song.