As with so many concepts in the social sciences, the definition of the term City is fuzzy and dependent upon the context in which the term is used. This essay covers some of the formal and informal ways in which the boundaries of cities are defined, and the ways in which those boundaries contain and are contained by other boundaries. While the focus is on the United States, many of these concepts apply to urban areas around the world.

Cities as Governing Boundaries

A dictionary definition of a City is an inhabited place of greater size, population, or importance than a town or village. More specifically in the United States, it is a large or important municipality in the United States governed under a charter granted by the state.

That charter is a legal document that creates a Municipal Corporation, which is an incorporated political subdivision of a state that is composed of the citizens of a designated geographic area and which performs certain state functions on a local level and possesses such powers as are conferred upon it by the state. These corporations have the power to create governing bodies (such as mayors, city councils, etc.) that are elected by residents of the municipality. Those governments have the power to pass and enforce (police) local laws, and conduct business (such a building infrastructure).

The US Census bureau refers to areas contained by municipal corporations as Incorporated Places and the formation of a municipal corporation is called Incorporation. While the exact legal process varies from state to state, this usually involves getting a majority of the voters in the potentially incorporated area to vote to incorporate.

Areas within cities can Secede from that city to form a separate city, and cities can Annex adjacent cities to form one larger city. Both of these processes are usually complex, politically difficult and involve lawyers.

Cities exist within a hierarchy of administrative boundaries. National (federal) and state laws apply within cities, and laws passed within cities can usually be overridden by laws passed at the federal or state level.

Counties are the primary legal divisions of most states. Most counties are functioning governmental units, whose powers and functions vary from state to state.

Minor Civil Divisions (MCD) are legal divisions of counties and can exist as governing units (with elected officials) or simply for administrative purposes (such as election districts). Not all states have MCDs, and the names used for these vary by state: town, township, district.

Although most cities have separate governments from their counties, some cities (such as Denver) combine their city and county or MCD into a single unit of government. Such cities are called Consolidated Cities. Denver is an example of a consolidate city.

If you want more specifics on the definitions of these geographic areas, see the US Census Bureau's page on Geographic Terms and Definitions.

Cities as Urban Areas

The term Urban is defined as relating to cities and the people who live in them. However, that usage of the word city is more qualitative than legal. It may be more useful to think of the concept of urban as forming a continuum with Rural at the other extreme.

Some New Urbanist city planners use a Transect that ranks areas on how rural or urban they are. On one extreme are rural areas with no people or buildings, and on the other extreme are dense urban areas with many people and tall buildings. The distinction primarily revolves around Density, or how many people and buildings are packed together in a given area.

Urban areas can usually be seen as encompassing multiple adjacent, but legally separate, incorporated cities. Such areas are referred to as Metropolitan Areas or Conurbations. Metropolitan areas usually have a Central City by which the metropolitan area is referred. For example, the Denver metropolitan area encompasses not only the central city of Denver, but cities like Aurora and Lakewood that have their own separate city governments.

The political divisions within metropolitan areas are almost always the result of decades of political battles. As cities grew in the 20th century, outlying communities often resisted annexation to avoid additional taxation or loss of control over their own affairs. The result in many metropolitan areas has been Municipal Fragmentation, leaving a metropolitan area with a large number of competing cities that often find it difficult to address common problems. For example, depending on how you count, the Denver metropolitan area is comprised of around 50 separate incorporated cities.

The process of urban areas expanding into widely dispersed, far-flung regions is often referred to as Suburban Sprawl. While sprawl provides affordable housing and comfortable lifestyles, sprawl exacts high costs on cities for building and maintaining infrastructure while also increasing the amount of traffic and associated environmental degradation.

Some analysts expand the concept of the metropolitan area even further into the idea of Megaregions. As metropolitan regions continued to expand throughout the second half of the 20th century their boundaries began to blur. Interlocking economic systems, shared natural resources and ecosystems, and common transportation systems link these population centers together.

One notable megaregion is what French Geographer Jean Gottman in 1954 called Megalopolis, the chain of overlapping metropolitan areas in the Northeast US from Boston, MA down to Washingon, DC. The term megalopolis actually dates back to the early 20th century and is today sometimes used to refer to any similarly large chain of metropolitan regions, such as the Pearl River Delta in China that is home to over 55 million people.

At an even larger scale is the concept of Global Cities that form the economic nodes of the global economy, and are home to major financial markets, multiple transnational corporations and/or global governance institutions. Global cities, in many ways, represent their own distinct virtual region as residents commonly live in or travel between global cities and have little interaction with the areas between (flyover states) or adjacent to these global cities.

While cities like Singapore, London and New York are univerally acknowledged to be global cities, exactly which smaller cities should be considered global is a matter of debate. This is a list of global cities from the perspective of the consulting firm A.T. Kearney

Note that the concept of a global city is distinct from that of a Megacity. Megacities are urban areas with 10 million or more residents. A city must be not only large, but be an globally-important economic center to be a global city. For example, Lagos, Nigeria is a sprawling, vibrant and growing city with over 16 million residents, but it has yet to achieve a great enough importance to be considered a global city.

One important global trend is a rapid increase in the percentage of the world's population that live in cities. While the majority of citizens in MDCs have long lived in urban areas, many LDCs in Africa and Asia are currently undergoing extremely rapid urbanization. In many cases, what Anna Tibaijuka in the UN State of the World's Cities called Premature Urbanization is occuring as rural residents move into cities (either pulled by opportunity or pushed out of desperation) before those urban economies are able to support them at anything close to the standards of the developed world.

Cities as Electoral Boundaries

Governing officials in democracies are commonly (but not always) elected by voters grouped into geographically defined areas, with the boundaries determined by complex (and, arguably, often corrupt) political processes.

Historically, there are five forms of municipal governments in the US:

Cities as Statistical Boundaries

Because politically-defined boundaries often do not clearly reflect the facts on the ground that demographers and social scientists seek to analyze, the US Census Bureau defines a number of different statistical areas associated with cities. Knowledge of these areas will be of value to you if you perform analysis of urban phenomena in your future work or research.

Cities are composed of Individuals and Households, but to preserve confidentiality, the Census Bureau does not publicly release individual data.

The smallest areas that the Census Bureau will release data on are Census Blocks and Census Tracts. Census tracts boundaries are usually drawn to contain around 4,000 people. Census tracts may be divided into two or more census blocks. Below are the census tracts in the City of Denver.

The Census Bureau defines an Urban Area as a densely settled core of census tracts and/or census blocks that meet minimum population density requirements. Below is the urban area surrounding the core City of Denver.

To encompass full metropolitan areas, the Census Bureau creates Core-Based Staistical Areas (CBSA). Metropolitan Statistical Areas (MSA) are CBSAs with a core urban area containing a substantial population nucleus, together with adjacent communities having a high degree of economic and social integration with that core. Below is the Denver-Aurora-Lakewood MSA.

Combined Statistical Areas (CSA) are CBSAs consisting of two or more adjacent MSAs that have substantial employment interchange. Below is the Denver-Aurora CSA

The US Census Bureau makes a wide variety of information about people and businesses in communities available on their American FactFinder web site.

Areas Within Cities

Cities commonly have a Central Business District (CBD), usually "downtown" where major financial and governmental functions occur.

In many US cities, CBDs evolved over the 20th century to be devoted almost exclusively to professional activity with few people actually living there. However, in recent years, housing has returned to some CBDs as many people increasingly find urban life and walkable cities attractive.

Many cities and metropolitan areas have evolved into Polycentric Cities that have multiple centers of commercial activity beyond or in place of a CBD. Los Angeles is a prime example of a polycentric city.

Suburban development has resulted in Edge Cities with concentrations of business, shopping and entertainment created in formerly residential areas. Edge cities often are developed around expressway exits, providing convenient access to patrons from surrounding residential communities.

Cities frequently control what kinds of activities can go on in different parts of the city, especially distinguishing between residential areas and different types of commercial activity. Zoning involves defining these areas. Zoning arose in the USA over the 20th century as a means to keep residences separate from dirty industrial activities. It is also a highly contentious process that has been used to segregate classes of people and preserve private property values.

Neighborhoods are areas within cities with some kind of unifying identity. Neighborhoods may be named by government officials or private developers, or can be the legacy of historic national or ethnic affinities, such as areas where immigrants from specific countries or ethnic groups settled. Because neighborhood boundaries reflect ever-changing human settlement and interaction, the boundaries of neighborhoods are often ambiguous and fluid. While city agencies do sometimes provide specific geospatial boundaries for neighborhoods, these are often approximations made by geographers seeking to impose an order for analytical purposes.

There are numerous other divisions within cities. For example:

Physical Area, Population and Population Density

Two fundamental characteristics of a city are the Population (the number of people who live there) and the amount of physical Area that city covers.

Dividing the population by the area gives a Population Density, expressed as residents per square mile or square kilometer. Whether density is good or bad depends on what lifestyle conditions you value. Many people in MDCs with access to private transportation and personal wealth prefer low-density development, with large houses and ample open space. In contrast, many people in LDCs have few options other than to live in, often dangerous, areas of high population density.

While population density is a convenient metric for judging how crowded a city is, a single number cannot fully capture the complex dynamics of life in a city. Areas with large numbers of commuters have daytime populations (and crowding) that are not reflected in residential population density. Densely-populated cities with robust public transit systems can have much more convenient lifestyles than sprawled, auto-dependent cities with chronic traffic congestion.

Decline and Renewal

Cities sporadically experience periods of redevelopment where older buildings are demolished and poor residents are displaced in efforts to make cities more economically productive.

Urban Renewal is a broad term for government programs to demolish large areas of old buildings and replacing them with newer buildings - often in an effort to provide housing and economic activity catered to wealthier residents. Urban renewal efforts were common in the US in the 1960s and 1970s, and while the term is rarely used today, such efforts still occur in the developing world.

Gentrification is a more subtle process where increasing urban property values and rents result in a gradual replacement of old buildings and poor residents with new buildings and wealthier residents.

In both urban renewal and gentrification, debates often arise between architectural preservation and modernization. Preservationists seek to preserve and maintain older architecture as a historical legacy, while Modernists seek to replace older architecture with contemporary architectural styles they feel are more suited to the modern world.