The question of determining the world's largest cities does not allow a single, simple answer. It depends on which definitions of "city" and "size" are used, and how those definitions are applied. Complex political/cultural/social situations, sometimes controversial or disputed, further confuse the discussion. Debate on this field is highly vulnerable to bias or manipulation, as people tend to prefer whichever definition most flatters their own city.

The "size" of a city can refer to either its land area or, more typically, its population.

The borders of a city can also be defined in any of several ways:

Morphological

"City" defined as a physically contiguous urban area, without regard to territorial or other boundaries. The delineation is usually done using some type of urban density, such as population density or density of buildings (for example, "gaps between structures may not exceed 200 metres"). Satellite and/or aerial maps may be used. For statistical convenience, such areas are sometimes adjusted to appropriate administrative boundaries, yielding an agglomeration.

Functional

"City" as defined by the habits of its demographic population, as by metropolitan area, labor market area, or similar. Such definitions are usually based on commuting between home and work. Commuter flow thresholds into the core urban area are established by the national census authority, determining which areas are included.

Administrative

"City" as strictly defined by a given government (city proper). Typically based on a municipality or equivalent entity, or sometimes a group of municipalities under a regional government.