NINE THOUSAND years ago the Sahara desert was a grassland, inhabited by hunters who made rock paintings of hippos and giraffes. For a millennium before the 16th century a flourishing trade carried salt, gold and slaves across the dunes, until Moroccan invasions and Atlantic shipping drove it into decline. Yet today the Sahara is more often seen as a barrier, cutting Africa in two. Academics, policymakers and newspapers—including The Economist—routinely refer to part of the continent as “sub-Saharan Africa”. International institutions such as the World Bank and IMF are internally organised along the same lines. It seems Africa is defined by a wall of sand. But what is “sub-Saharan Africa”?

The answer might seem obvious. Anywhere south of the desert is, geographically, “sub-Saharan”. The first problem is that some countries, like Mauritania, are mostly in the desert itself. And the confusion runs deeper. Consider Somalia and Djibouti, both in the Horn of Africa. They are south of the Sahara, but the IMF oversees them from its Middle East and Central Asia department. The World Bank used to include both countries in sub-Saharan Africa, before moving Djibouti to the Middle East and North Africa in 2000. Meanwhile Eritrea, to the north of both of them, is considered sub-Saharan. And whereas the World Bank includes the Arabic-speaking states of Mauritania and Sudan in sub-Saharan Africa, the IMF does not.

Clearly, “sub-Saharan Africa” is not just about physical geography, but has a cultural element too. Outsiders have often regarded the region with a mixture of ignorance, fantasy and racism. “Geographers in Afric-Maps, with Savage-Pictures fill their Gaps,” scoffed Jonathan Swift. Arab writers referred to the region south of the Sahara as bilad al-sudan, or “land of the blacks”. The term was used to describe a larger area than modern-day Sudan, stretching roughly from Senegal to Ethiopia. Some 18th-century British mapmakers simply translated it as “Negroland”. Colonial administrators favoured the phrase “tropical Africa” for everything between the Sahara and the Limpopo river. In the 1970s the term “black Africa” became popular among scholars, including some in Africa. It was only in the 1980s that the phrase “sub-Saharan Africa” gained purchase, though it began circulating earlier (The Economist first used it in 1938). All of these concepts were entangled with notions of race, language and the level of economic development. In some cases this has been painfully obvious. In the 1960s and 1970s, the World Bank lumped white-ruled, apartheid-era South Africa together with the Middle East and North Africa. In the late 1990s, when the country was governed by a black majority government, and was once more receiving World Bank loans, it appeared in the sub-Saharan category.

Many in Africa reject regional labels altogether. African-led institutions, such as the African Union (AU) and the African Development Bank, encompass the whole continent, up to the Red Sea. They stand in a tradition stretching back to the 1960s, when pan-Africanism was at its peak. Ali Mazrui, a Kenyan intellectual, later wrote about “Afrabia”, arguing that Africa and the Arab world “are in the slow historic process of becoming one”. Some pan-Africanists cast the net still wider, to include the black Caribbean. These expansive definitions can quickly become bewildering. Binyavanga Wainaina, a Kenyan writer, laments the Western tendency to “treat Africa as if it were one country”, something that may explain why tourism drops in Botswana when Ebola hits Sierra Leone. Yet some trends—from the spread of cross-border business to the geography of foreign aid—can only be understood on a regional scale. So “sub-Saharan Africa” will continue to appear, including in the pages of The Economist. Even if it is a little fuzzy around the edges.