General definition

A metropolitan area usually combines an agglomeration (the contiguous built-up area) with peripheral zones not themselves necessarily urban in character, but closely bound to the center by employment or commerce; these zones are also sometimes known as a commuter belt, and may extend well beyond the urban periphery depending on the definition used. It is mainly the area that is not part of the city but is connected to the city. For example, Pasadena, California would be added to Los Angeles, California's metro area. While it isn't the same city, it is connected.

The core cities in a polycentric metropolitan area need not be physically connected by continuous built-up development, distinguishing the concept from conurbation, which requires urban contiguity. In a metropolitan area, it is sufficient that central cities together constitute a large population nucleus with which other constituent parts have a high degree of integration.

In practice the parameters of metropolitan areas, in both official and unofficial usage, are not consistent. Sometimes they are little different from an urban area, and in other cases they cover broad regions that have little relation to the traditional concept of a city as a single urban settlement. Thus all metropolitan area figures should be treated as interpretations rather than as hard facts. Metro area population figures given by different sources for the same place can vary by millions, and there is a tendency for people to promote the highest figure available for their own "city". However the most ambitious metropolitan area population figures are often better seen as the population of a "metropolitan region" than of a "city". fact: date=September 2008

Differences in terminology by country

The term metropolitan area is sometimes abbreviated to 'metro', for example in Metro Manila and Washington, DC Metro Area, which in the latter case should not be mistaken to mean the metro rail system of the city. Although it can be compared in composition to many of the world's metropolitan areas, in France the term for the region around an urban core linked by commuting ties is an aire urbaine (officially translated as "urban area"). In Japan that would be .

Australia

United States

The Office of Management and Budget defines "Core Based Statistical Areas" used for statistics purposes among federal agencies. Each CBSA is based on a core urban area and is composed of the counties which comprise that core as well as any surrounding counties that are tightly socially or economically integrated with it. These areas are designated as either metropolitan or micropolitan statistical areas, based on population size; a "metro" area has an urban core of at least 50,000 residents, while a "micro" area has less than 50,000 but at least 10,000.