Historically, city-dwellers have been a small proportion of humanity overall, but following two centuries of unprecedented and rapid urbanisation, roughly half of the world population now lives in cities, which has had profound consequences for global sustainability. Present-day cities usually form the core of larger metropolitan areas and urban areas—creating numerous commuters traveling towards city centres for employment, entertainment, and edification. However, in a world of intensifying globalisation, all cities are in different degree also connected globally beyond these regions. This increased influence means that cities also have significant influences on global issues, such as sustainable development, global warming and global health.
A city is distinguished[by whom?] 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. The term can also refer either to the physical streets and buildings of the city or to the collection of people who dwell there, and can be used in a general sense to mean urban rather than rural territory.
National censuses use a variety of definitions - invoking factors such as population, population density, number of dwellings, economic function, and infrastructure - to classify populations as urban. Typical working definitions for small-city populations start at around 100,000 people. Common population definitions for an urban area (city or town) range between 1,500 and 50,000 people, with most U.S states using a minimum between 1,500 and 5,000 inhabitants. Some jurisdictions set no such minima. In the United Kingdom, city status is awarded by the Crown and then remains permanently. (Historically, the qualifying factor was the presence of a cathedral, resulting in some very small cities such as Wells, with a population 12,000 as of 2018 and St Davids, with a population of 1,841 as of 2011.) 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. Cities serve as administrative, commercial, religious, and cultural hubs for their larger surrounding areas. Examples of settlements with "city” in their names which may not meet any of the traditional criteria to be named such include Broad Top City, Pennsylvania (population 452), and City Dulas, Anglesey, a hamlet.
The presence of a literate elite is sometimes included[by whom?] in the definition. A typical city has professional administrators, regulations, and some form of taxation (food and other necessities or means to trade for them) to support the government workers. (This arrangement contrasts with the more typically horizontal relationships in a tribe or village accomplishing common goals through informal agreements between neighbors, or through leadership of a chief.) The governments may be based on heredity, religion, military power, work systems such as canal-building, food-distribution, land-ownership, agriculture, commerce, manufacturing, finance, or a combination of these. Societies that live in cities are often called civilizations.