World's largest cities

Determining the world's largest cities depends on which definitions of city are used. The United Nations uses three definitions for what constitutes a city, as not all cities may be classified using the same criteria. Cities may be defined as the cities proper, the extent of their urban area, or their metropolitan regions.

A city can be defined by its administrative boundaries (city proper). World Urbanization Prospects, a UN publication, defines population of a city proper as "the population living within the administrative boundaries of a city or controlled directly from the city by a single authority." UNICEF defines city proper as "the population living within the administrative boundaries of a city, e.g., Washington, D.C.". A city proper is a locality defined according to legal or political boundaries and an administratively recognised urban status that is usually characterised by some form of local government. Cities proper and their boundaries and population data may not include suburbs.

The use of city proper as defined by administrative boundaries may not include suburban areas where an important proportion of the population working or studying in the city lives. Because of this definition, the city proper population figure may differ greatly with the urban area population figure, as many cities are amalgamations of smaller cities (Australia), and conversely, many Chinese cities govern territories that extend well beyond the traditional "city proper" into suburban and rural areas.

A city can be defined as a conditionally urban 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 city can be defined by the habits of its demographic population, as by metropolitan area, labour market area, or similar in a metropolitan area. UNICEF defines metropolitan area as follows:

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