Strictly speaking, there’s no official definition of a suburb. The U.S. Census Bureau tries to explain it this way:
[QUOTE=Fuzzy Bureaucrat-speak]
The general concept of a metropolitan area (MA) is one of a large population nucleus, together with adjacent communities that have a high degree of economic and social integration with that nucleus. Some MA’s are defined around two or more nuclei.
The MA classification is a statistical standard, developed for use by Federal agencies in the production, analysis, and publication of data on MA’s. The MA’s are designated and defined by the Federal Office of Management and Budget, following a set of official published standards. These standards were developed by the interagency Federal Executive Committee on Metropolitan Areas, with the aim of producing definitions that are as consistent as possible for all MA’s nationwide.
Each MA must contain either a place with a minimum population of 50,000 or a Census Bureau-defined urbanized area and a total MA population of at least 100,000 (75,000 in New England). An MA is comprised of one or more central counties, and an MA may also include one or more outlying counties that have closed economic and social relationships with the central county. An outlying county must have a specified level of commuting to the central counties and also must meet certain standards regarding metropolitan character, such as population density, urban population, and population growth. In New England, MA’s are composed of cities and towns rather than whole counties.
The territory, population, and housing units in MA’s are referred to as “metropolitan.” The metropolitan category is subdivided into “inside central city” and “outside central city.” The territory, population, and housing units located outside MA’s are referred to as “nonmetropolitan.”
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From this, one may infer that a suburb is “adjacent communities that have a high degree of economic and social integration with that nucleus.”
And, specifically to previous posters, what we call Phoenix, the Census Bureau refers to as Phoenix-Mesa-Scottsdale; San Fransisco is San Francisco-Oakland-Fremont; San Jose is considered a separate metropolitan area; and Kenosha is consolidated with Chicago but Racine is consolidated with Milwaukee.