A city is an area in which a large number of people live fairly close together. Cities usually have their own separate governments and systems for maintaining and providing utilities and transportation.
A city is basically a big town — the population is large in relation to the amount of land, since people often live in apartments or multi-family housing. The largest city in the world today is Shanghai, China. You can also call the residents of a city as a whole a city: "The city voted to increase recycling." City comes from the Latin civitatem, "citizenship," or "community of citizens," from the root civis, "citizen."