Determining the world's largest cities depends on which definitions of "city" and "size" are used, and how those definitions are applied. The "size" of a city can refer to its land area, but it is more typically the population. How one defines the land area of a city, however, is key to determining its population.
A city can be defined as a conditionally contiguousurban area, without regard to territorial or other boundaries inside an urban area. UNICEF defines urban area as follows:
The definition of "urban" varies from country to country, and, with periodic reclassification, can also vary within one country over time, making direct comparisons difficult. An urban area can be defined by one or more of the following: administrative criteria or political boundaries (e.g., area within the jurisdiction of a municipality or town committee), a threshold population size (where the minimum for an urban settlement is typically in the region of 2,000 people, although this varies globally between 200 and 50,000), population density, economic function (e.g., where a significant majority of the population is not primarily engaged in agriculture, or where there is surplus employment) or the presence of urban characteristics (e.g., paved streets, electric lighting, sewerage).
A formal local government area comprising the urban area as a whole and its primary commuter areas, typically formed around a city with a large concentration of people (i.e., a population of at least 100,000). In addition to the city proper, a metropolitan area includes both the surrounding territory with urban levels of residential density and some additional lower-density areas that are adjacent to and linked to the city (e.g., through frequent transport, road linkages or commuting facilities).