A developing country, also called a less developed country or underdeveloped country, is a nation with an underdeveloped industrial base, and a low Human Development Index (HDI) relative to other countries. On the other hand, since the late 1990s developing countries tended to demonstrate higher growth rates than the developed ones. There is no universal, agreed-upon criterion for what makes a country developing versus developed and which countries fit these two categories, although there are general reference points such as a nation's GDP per capita compared to other nations. Also, the general term less-developed country should not be confused with the specific least developed country.

The term Third World arose during the Cold War to define countries that remained non-aligned with either NATO, or the Communist Bloc. The United States, Western European nations and their allies represented the First World, while the Soviet Union, China, Cuba, and their allies represented the Second World. This terminology provided a way of broadly categorizing the nations of the Earth into three groups based on social, political, cultural and economic divisions. The Third World was normally seen to include many countries with colonial pasts in Africa, Latin America, Oceania and Asia. It was also sometimes taken as synonymous with countries in the Non-Aligned Movement. In the dependency theory of thinkers like Raul Prebisch, Walter Rodney, Theotonio dos Santos, and Andre Gunder Frank, the Third World has also been connected to the world economic division as "periphery" countries in the world system that is dominated by the "core" countries.