The definition of an exurb is an area beyond the suburbs where the wealthy people live.(noun)
A wealthy area where people live outside of the suburbs of Philadelphia is an example of an exurb.
See exurb in Webster's New World College Dictionary
Origin: ex- + (sub)urb: coined (1955) by A. C. Spectorsky (1910-72), U.S. author and editor
See exurb in American Heritage Dictionary 4
Origin: + (sub)urb.