The Sahel is the ecoclimatic and biogeographic zone of transition between the Sahara desert in the North and the Sudanian Savannas in the south. It stretches across the north of the African continent between the Atlantic Ocean and the Red Sea. The Arabic word sāḥil ساحل literally means "shore, coast", describing the appearance of the vegetation of the Sahel as a coastline delimiting the sand of the Sahara.

The Sahel covers parts of the territory of (from west to east) Senegal, southern Mauritania, Mali, Burkina Faso, southern Algeria, Niger, northern Nigeria, Chad, northern Cameroon, Sudan (including Darfur and the southern part of Sudan), and Eritrea. It is bordered on the north by the Sahara and on the south by the less arid savannah.

The Sahel is 5,400 km (3,400 mi) from the Atlantic Ocean in the west to the Red Sea in the east, in a belt that varies from several hundred to a thousand kilometers (620 miles) in width, covering an area of 3,053,200 square kilometers (1,178,800 sq mi). It is a transitional ecoregion of semi-arid grasslands, savannas, steppes, and thorn shrublands lying between the wooded Sudanian savanna to the south and the Sahara to the north.

The Sahelian kingdoms were a series of kingdoms or empires that were centered on the sahel, the area of grasslands south of the Sahara. The wealth of the states came from controlling the trade routes across the desert. Their power came from having large pack animals like camels and horses that were fast enough to keep a large empire under central control and were also useful in battle. All of these empires were also quite decentralized with member cities having a great deal of autonomy.

The Sahel states were limited from expanding south into the forest zone of the Ashanti and Yoruba as mounted warriors were all but useless in the forests and the horses and camels could not survive the heat and diseases of the region.