What makes a city?

[RedSLC]

It's tempting to say 'a large urban area', but that's not actually right, is it? Hereford (pop. c. 59,000) is most certainly a city, while Dudley (which, if we take a pretty wide - and pretty absurd - definition of 'Dudley', contains more than 300,000 souls) is quite clearly not. And wouldn't really be one even if (God forbid) it somehow flukes city status at some point.

Check out Dudley for yourself on this aerial photograph . I tend to say it actually shows up as a separate light point, which - if I understand Torie's approach correctly - would make it a city (though it is clearly a borderline case).
The whole "urban area" or "agglomeration" point does not bring us further here - the Ruhr/ Rhine, the Midlands and the Dutch Randstad can all be identified as polycentric urban areas/ agglomerations, made up of several distinct city cores - just as you can tell from air that NYC and Philadelphia, while growing together, are still two separate cities.

The cases that are really complicated are San Francisco / Oakland / San Jose, Athens/ Piraeus, even Berlin / Potsdam (aerial photograph on the latter two). Shall we really stop calling Piraeus a polis, just because it has from air become indistinguishable from Athens?

I'd worry about leaning too much on light generation as a measure of cityness. Otherwise Williston, ND becomes a major US city.

Lol. The cluster of light around Williston is bigger than the tiny strip of light that I live in.

[RedSLC]

IMO, what distinguishes a city from a suburb or other similarly-sized unit is whether it has well-developed downtown area, where a lot of people from the surrounding areas work. i.e. it has to be more than just retail outlets, real estate offices, etc. that are usually found in the middle of suburbs.