A city is a large and permanent human settlement. Although there is no agreement on how a city is distinguished from a town in general English language meanings, many cities have a particular administrative, legal, or historical status based on local law.
Cities generally have complex systems for sanitation, utilities, land usage, housing, and transportation. The concentration of development greatly facilitates interaction between people and businesses, sometimes benefiting both parties in the process, but it also presents challenges to managing urban growth.
A big city or metropolis usually has associated suburbs and exurbs. Such cities are usually associated with metropolitan areas and urban areas, creating numerous business commuters traveling to urban centers for employment. Once a city expands far enough to reach another city, this region can be deemed a conurbation or megalopolis. Damascus is arguably the oldest city in the world. In terms of population, the largest city proper is Shanghai, while the fastest-growing is Dubai.
The conventional view holds that cities first formed after the Neolithic revolution. The Neolithic revolution brought agriculture, which made denser human populations possible, thereby supporting city development. The advent of farming encouraged hunter-gatherers to abandon nomadic lifestyles and to settle near others who lived by agricultural production. The increased population density encouraged by farming and the increased output of food per unit of land created conditions that seem more suitable for city-like activities. In his book, Cities and Economic Development, Paul Bairoch takes up this position in his argument that agricultural activity appears necessary before true cities can form.
“What strange phenomena we find in a great city, all we need do is stroll about with our eyes open. Life swarms with innocent monsters.”
― Charles Baudelaire
According to Vere Gordon Childe, for a settlement to qualify as a city, it must have enough surplus of raw materials to support trade and a relatively large population. Bairoch points out that, due to sparse population densities that would have persisted in pre-Neolithic, hunter-gatherer societies, the amount of land that would be required to produce enough food for subsistence and trade for a large population would make it impossible to control the flow of trade. To illustrate this point, Bairoch offers an example: 
“Western Europe during the pre-Neolithic, the density must have been less than 0.1 person per square kilometre”. Using this population density as a base for calculation, and allotting 10% of food towards surplus for trade and assuming that city dwellers do no farming, he calculates that “…to maintain a city with a population of 1,000, and without taking the cost of transport into account, an area of 100,000 square kilometres would have been required. When the cost of transport is taken into account, the figure rises to 200,000 square kilometres …”. Bairoch noted that this is roughly the size of Great Britain. The urban theorist Jane Jacobs suggests that city formation preceded the birth of agriculture, but this view is not widely accepted.
In his book City Economics, Brendan O’Flaherty asserts “Cities could persist—as they have for thousands of years—only if their advantages offset the disadvantages. O’Flaherty illustrates two similar attracting advantages known as increasing returns to scale and economies of scale, which are concepts usually associated with businesses. Their applications are seen in more basic economic systems as well. Increasing returns to scale occurs when “doubling all inputs more than doubles the output an activity has economies of scale if doubling output less than doubles cost”. To offer an example of these concepts, O’Flaherty makes use of “one of the oldest reasons why cities were built: military protection” .