What about the rural?

U.S. Census Bureau defines rural as any population, housing, or territory NOT in an urban area. So by its definition, rural is not a self-defined condition, but it is the absence of the urban. Then it goes forward to define urbanized areas as having a population of 50,000 or more. The green area on the map above (US Census) represents all the area in the US that falls under the classification of rural based on this definition.