when is a city a city?

is a whole bunch of big and impressive buildings put together a city? or does it feature other things? is the mark of a city in its infrastructure? roads, traffic systems, street signs, etc. or can you have a city without all of them? if there is no urban planning, is there no metropolis? and what about the people? can you have a city without buildings at all, just citizens?

again, inspired by the rmit architecture studio, where there was a megalopolis of models, all on the one table. it just got me wondering, that's all.