A megacity is a very large city, typically with a population of more than 10 million people. The United Nations Department of Economic and Social Affairs in its "World Urbanization Prospects" report defines megacities as urban agglomerations with over 10 million inhabitants. Reports from academics, the European Council, the Organisation for Economic Co-operation and Development delineate them as the metropolitan areas that are inhabited by more than a total population of 10 million people. The fast growth of the former is attributed to the proliferation of the latter. Infrequently in some sources, from five to eight million is considered the minimum threshold, along with a population density of at least 2,000 per square kilometre. The terms conurbation, metropolis, and metroplex are also applied to the latter.