The term "urban" includes land areas, populations and housing developments located in urbanized areas, such as cities and towns or urban clusters, while "rural" refers to territory, populations and housing units located outside urbanized areas or clusters. Urban areas or urban centers feature densely populated landscapes, and typically have census-determined population densities of more than 1,000 residents per square mile. Rural areas, in contrast, generally have fewer than 500 people per square mile, and fall outside the borders of urbanized areas, urban clusters and their surrounding census blocks.
In many locations, there is an area of semi-dense populations surrounding urbanized areas and urban centers. These areas form the suburbs or outskirts of more populated areas, such as towns and cities. These outer areas are classified as urban if they have population densities exceeding 500 inhabitants per square mile. Rural areas include regions beyond urban centers and their outskirts that have fewer than 500 residents per square mile. Sometimes, there is an overlap between urban and rural areas. In other places, census data shows a borderline designation of classification as rural or urban. These gray areas may be grouped into the categories of either urban or rural, depending on the possibility for future growth in the surrounding areas.