From The Art and Popular Culture Encyclopedia
The Deep South is a descriptive category of cultural and geographic subregions in the American South. Historically, it is differentiated from the "Upper South" as being the states which were most dependent on plantation type agriculture during the antebellum period. The Deep South was also commonly referred to as the Lower South or the "Cotton States".
Today, the Deep South is usually delineated as being those states and areas where things most often thought of as "Southern" exist in their most undiluted and concentrated form.