What is a City?

Does a city actually exist as we think about it or is this ambiguous thing that ever changes with the population and location? As I was reading it seemed that what made up a city was constantly changing based of the increase in density and the technology and innovation changes that occur in order to support the new found population. There was the elevator which revolutionized how high we could reasonable build where people would be willing to live and inhabit. They are these constructs that are formed in our mind and thinking which we then overlay onto places and as we grow and experience more, that way of thinking can change and evolve to match new ideas. Cities are dynamic entity however they tend to be defined by people and the density with in a region which if thought about is very arbitrary.  What really makes this a city and one a town?