The proportion of urban (and rural) population is estimated from the most recently available census or official population estimate of each country. If this estimate is only available for some period in the past, the proportion urban is extrapolated to the base year. In the 2018 Revision of the World Urbanization Prospects the base year is 2018.
The United Nations does not use their own definition of “urban” population but follows the definition that is used in each country. The definitions are generally those used by national statistical offices in carrying out the latest available census. When the definition used in the latest census was not the same as in previous censuses, the data were adjusted whenever possible so as to maintain consistency. In cases where adjustments were made, that information is included in the sources listed online. United Nations estimates and projections are based, to the extent possible, on actual enumerations. In some cases, however, it was necessary to incorporate other estimates of urban population size. When that is done, the sources of data indicate it.Source: UN World Urbanization Prospects