In the study of human settlements, an urban agglomeration is an extended city or town area comprising the built-up area of a central place (usually a municipality) and any suburbs linked by continuous urban area. INSEE, the French Statistical Institute, uses the term unité urbaine, which means continuous urbanized area. However, because of differences in definitions of what does and does not constitute an "agglomeration", as well as variations and limitations in statistical or geographical methodology, it can be problematic to compare different agglomerations around the world. It may not be clear, for instance, whether an area should be considered to be a satellite and part of an agglomeration, or a distinct entity in itself.
The term "agglomeration" is also linked to "conurbation", which is a more specific term for large urban clusters where the built-up zones of influence of distinct cities or towns are connected by continuous built-up development (Essen - Dortmund and others in the Rhine-Ruhr district), even in different regions, states or countries, (Lille - Kortrijk in France and Belgium). Each city or town in a conurbation may nevertheless continue to act as an independent focus for a substantial part of the area.
Largest occurrences 
The term "agglomeration" is more appropriate for determining the populations of large super-cities like Tokyo, New York City, Mexico City, Delhi, and Seoul. These examples are super-cities which have expanded enough to consume other neighborhoods. Although administration may be separate for outlying districts, the population might say they are from the central city. There are various lists of agglomeration populations. Three were presented here, one is left.
The 10 largest agglomerations by population in April 2011, according to Demographia:
|7||New York City||United States||20,710,000|
|10||Shanghai||People's Republic of China||18,665,000|
Specific legal definitions 
An agglomeration, or urban agglomeration, is an administrative subdivision of Quebec at the local level that may group together a number of municipalities which were abolished as independent entities on 1 January 2002 but reconstituted on 1 January 2006.
The eleven agglomerations of Quebec, under this legal definition, range in population from 2,000 to 2 million. In fact, only the agglomerations of Montreal, Quebec City, and Longueuil exceed 120,000 inhabitants.
Agglomerations are administered by agglomeration councils, and comprise a central municipality as well as numerous related municipalities, and their powers are defined by the Act respecting the exercise of certain municipal powers in certain urban agglomerations. Agglomerations have certain powers which, elsewhere in Quebec, are exercised by individual municipalities. (These powers are not the same as those of the metropolitan communities of Montreal and Quebec, which cover geographically larger areas than the corresponding agglomerations.) In some cases, agglomerations also have powers ordinarily exercised by regional county municipalities.
See also 
- World's largest cities - A list of the largest municipalities (cities) areas by population
- Metropolitan area
- Transborder agglomeration
- Urban sprawl
- Global city
- United Nations World Urbanization Prospects, the 2009 Revision, Web Site of the United Nations Population Division: Data on Urban Agglomerations
- e-Geopolis project Research Group, University Paris-Diderot
- Principal Agglomerations of the World
-  Demographia