Southern Illinois
While Southern Illinois is considered by people from Chicago to be anything south of I-80, Southern Illinois is pretty much anything south of Saint Louis. Highway 50 divides the lower tip of Illinois from the rest of the corn fields (and Chicago) and it starts across the river from Saint Louis so pretty much anything south of Highway 50 on a map is Southern Illinois. Cities in Southern Illinois include Belleville, East Saint Louis, Edwardsville, Carbondale, and not really much else.
The asshole of Southern Illinois is the Fairview/Shiloh/O'Fallon conglomeration of homogenized suburbia. This area tends to be full of soccer moms and SUV drivers. It is advised to avoid this area at all costs if one has any semblance of a soul. People from these areas tend to look down upon the more racially integrated cities of Belleville and Edwardsville.