From the Department of Agriculture:
Researchers and policy officials employ many definitions to distinguish rural from urban areas, which often leads to unnecessary confusion and unwanted mismatches in program eligibility. However, the existence of multiple rural definitions reflects the reality that rural and urban are multidimensional concepts. Sometimes population density is the defining concern, in other cases it is geographic isolation. Small population size typically characterizes a rural place, but how small is rural? Population thresholds used to differentiate rural and urban communities range from 2,500 up to 50,000, depending on the definition.
Because the U.S. is a nation in which so many people live in areas that are not clearly rural or urban, seemingly small changes in the way rural areas are defined can have large impacts on who and what are considered rural. Researchers and policymakers share the task of choosing appropriately from among alternate rural definitions currently available or creating their own unique definitions.