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A suburb is a mixed-use or residential area , existing either as part of a city or urban area or as a separate residential community within commuting distance of a city . In most English-speaking countries , suburban areas are defined in contrast to central or inner- city areas, but in Australian English and South African English, suburb has become largely synonymous with what is called a "neighborhood" in other countries and the term extends to inner- city areas. In some areas, such as Australia , China , New Zealand , the United Kingdom , and a few U.S. states, new suburbs are routinely annexed by adjacent cities . In others, such as Saudi Arabia , Canada , France , and much of the United States , many suburbs remain separate municipalities or are governed as part of a larger local government area such as a county. Suburbs first emerged on a large scale in the 19th and 20th centuries as a result of improved rail and road transport , which led to an increase in commuting. In general, they have lower population densities than inner city neighborhoods within a metropolitan area , and most residents commute to central cities or other business districts; however, there are many exceptions, including industrial suburbs, planned communities, and satellite cities . Suburbs tend to proliferate around cities that have an abundance of adjacent flat land .