WHAT WIKIPEDIA SAYS ABOUT URBAN AREAS

The UN uses three definitions for what constitutes a city, as not all cities in all jurisdictions are 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

A city can be defined as a conditionally contiguous urban area, without regard to territorial or other boundaries inside an urban area.

A city can be defined by the inhabitants of its demographic population, as by metropolitan area or labour market area.

OPINION

Although Wikipedia is not the original source, it offers the value that different international entities have calculated for an essential piece of information to know and analyze cities and their metropolitan area, such as population.