I was reading about the old Empire of Mali, and it made constant references to it being the dominant political power in the western part of the Sudan, which confused me as I thought Sudan was on the eastern coast by Ethiopia. It turns out there are two separate geographical entities which go by the same name, and I was wondering if there is any history which links together the names.
The etymological origins of the term Sudan comes from the Arabic phrase Bilad al-Sudan, which was a geographical term first used in the middle ages that encompassed Africa directly south of the Sahara desert, spanning from the Atlantic to the Red Sea. Bilad al-Sudan translates from arabic as "Lands of Blacks", in reference to the skin tone of the peoples living in the region.
So, in its original meaning, the Sudan region included areas of West Africa, the Lake Chad region and the upper Nile valley. So, the modern Republic of Sudan is an eastern portion of the historical Sudan region.
The kingdoms of the Middle Niger (Ghana, Gao, Mali, Songhai, Mossi, etc) are often categorized by scholars as being in the Western Sudan. Scholars continue to use this Arab geographical terminology because much of the earliest writing about the area comes from Arab traders and geographers.
Scholars sometimes also categorize the kingdoms of northern Nigeria, Niger, Chad and the Darfur region (Hausa states, Sokoto caliphate, Kanem-Borno, Wadai, Dar Fur) as being in the Central Sudan. The Central Sudan tends not to be as familiar a term to the public, simply because people tend not to hear about the polities of the Central Sudan, while they will have heard about the Mali empire and can find the Republic of Sudan on a map.