The name "Sudan" comes from the Arabic Bilād as-Sūdān (بلاد السودان), meaning "Land of the Blacks" — a geographic descriptor used by medieval Arab geographers for the vast Sahel region south of the Sahara.
Arab traders and scholars from the 9th–10th centuries onward used sūdān (plural of aswad, "black") to describe the dark-skinned peoples of the sub-Saharan belt stretching from West Africa to the Red Sea. The term appears in works by al-Ya'qubi (9th c.), al-Mas'udi (10th c.), and Ibn Khaldun (14th c.). It was geographic, not initially political.
Some scholars link the name to Soba, capital of the medieval Nubian kingdom of Alwa (c. 600–1500 CE) near modern Khartoum. The argument: Arabic speakers may have associated the region with its most prominent city. However, linguistic evidence for this derivation is thin.
Sawād (سواد) means "black land" and was used for the fertile, dark-soiled Nile Valley in Egypt and Iraq. Some suggest sūdān is a related formation emphasizing the people rather than the soil. Plausible, but the descriptive theory remains stronger.
"Sudan" described a zone, not a state, for centuries. The modern Republic of Sudan (est. 1956) inherited the name from the Anglo-Egyptian Condominium (1899–1956), which administered "the Sudan" as a defined territory. South Sudan (2011) kept the root name upon independence.
So: a medieval geographic label → a colonial territory → two modern nations. The name reflects how outsiders saw the region long before its current borders existed.