A variety of definitions, invoking population, population density, number of dwellings, economic function, and infrastructure, are used in national censuses to classify populations as urban. In the United Kingdom, city status is awarded on local criteria.Common population definitions for a city range between 1,500 and 50,000 people, with most U. According to the "functional definition" a city is not distinguished by size alone, but also by the role it plays within a larger political context.Access to water has long been a major factor in city placement and growth, and despite exceptions enabled by the advent of rail transport in the nineteenth century, through the present most of the world's urban population lives near the coast or on a river.The vast majority of cities have a central area containing buildings with special economic, political, and religious significance.The most populated city proper is Shanghai A city is distinguished from other human settlements by its relatively great size, but also by its functions and its special symbolic status, which may be conferred by a central authority.
Urban structure generally follows one or more basic patterns: geomorphic, radial, concentric, rectilinear, and curvilinear.
In a radial structure, main roads converge on a central point.
This form could evolve from successive growth over a long time, with concentric traces of town walls and citadels marking older city boundaries.
Their density facilitates interaction between people, government organizations and businesses, sometimes benefiting different parties in the process.
Historically, city-dwellers have been a small proportion of humanity overall, but following two centuries of unprecedented and rapid urbanization, roughly half of the world population now lives in cities, which has had profound consequences for global sustainability.