Comment by dredmorbius

Demographics and geography approaches this through the notion of "metropolitan statistical area" (MSA) in the US, or equivalent concepts elsewhere. An MSA is defined as a "geographical region with a relatively high population density at its core and close economic ties throughout the region".

<https://en.wikipedia.org/wiki/Metropolitan_statistical_area>

Other rough equivalents are metropolitan areas (UK), census metropolitan areas (Canada), functional urban areas (FUA) (EU), urban agglomerations (India). All of these use functional and behavioural characteristics to get around simple boundary-demarkation. Urbanisations may even span national borders, as with the greater Basel region, Lake Constance, or Strasbourg-Ortenau.