The first polis was in Ancient Greek, and the word described a city-state, an entity which included the means of government of the region, that kind of thing. Those were violent times, and the polis usually had a fortified area in the middle that was called the acropolis (which means high city).
There are still plenty of polises about nowa days. A megalopolis (that means big city) will probably be a merger of several smaller cities. A metropolis (mother city) is probably a seat of government (sweetly, there's a word minimetropolis, too). A tripolis is a group of three cities (Tripoli is one of those - in fact Tripoli is two of those, because there's another one in Lebanon). A similar idea gives us pentapolis, dodecapolis, duododecapolis. An eperopolis is a landmass city, one so big it takes up most of a country. An ecumenopolis is a city that takes up most of a planet.
Then we have cosmopolis (world city (though this is not so much to do with size as diversity)) a necropolis (city of the dead, a cemetery with lots of tombs the size of houses, basically) propolis (a before-city, which can mean either a suburb, or the stuff bees use to glue their hives together) exopolis (outside city: a modern word for urban sprawl) Heliopolis (sun city: they worshipped the sun there), ideopolis (knowledge city) and technopolis (one that makes hi-tech stuff).
So there we are.
Basically, I suppose you'd have to say it's mostly marketing.
Word To Use Today: one ending polis.