Define urban, please. At what population density is something "urban" vs. "rural"? The Phoenix metro is /huge/, but much of it is considered to be "rural" by th

Define urban, please. At what population density is something "urban" vs. "rural"? The Phoenix metro is /huge/, but much of it is considered to be "rural" by the locals there because it doesn't come even slightly close to the density of a real city like SF or NYC.