The U.S. Census Bureau does not define the term “exurb.” But it thinks of them “as far outer suburbs of metro areas, and they often have a mix of urban and rural character.”

Exurbs are attracting attention now because they are among the nation’s fastest-growing communities, according to Census data, even though many are located “30, 40 and even more than 60 miles away from the largest city’s downtown.”

An analysis by StorageCafe puts more meat on the Census’s bare-bones description. It defines exurbs more broadly as areas with fewer than 250 residents per square mile in the first year of its 10-year analysis. It found that housing inventory in exurbs grew by an average of 15% over the decade, outpacing suburbs at 14% and principal cities at 10%. Population growth at 16% also outpaced suburbs (13%) and principal cities (9%).