A megacity is a very large city, typically with a population of more than 10 million people.[1][2] Precise definitions vary: the United Nations Department of Economic and Social Affairs in its 2018 "World Urbanization Prospects" report counted urban agglomerations having over 10 million inhabitants.[3] A University of Bonn report held that they are "usually defined as metropolitan areas with a total population of 10 million or more people".[4] Others list cities satisfying criteria of either 5 or 8 million and also have a population density of 2,000 per square kilometre.[5] The terms conurbation, metropolis, and metroplex are also applied to the latter.[5]

The total number of megacities in the world varies between different sources: The world had 33 according to the UN (in 2018), 37 according to CityPopulation.de (in 2020), and 35 according to Demographia (in 2020). Many of these urban agglomerations are in China and India. The other four countries with multiple megacities are the United States, Brazil, Pakistan, and Japan. African megacities are present in Nigeria, Egypt, and the DRC; European megacities are present in Russia, France, the United Kingdom, and Turkey (also in Asia); megacities can be found in Latin America in the countries of Brazil, Mexico, Colombia, Peru, and Argentina. Some sources identify Tokyo (Greater Tokyo Area) as the largest megacity in the world,[6][3] while some others give the title to Guangzhou (Pearl River Delta).[7][8][9]