Urban sprawl (also known as suburban sprawl or urban encroachment) is defined as "the rapid expansion of the geographic extent of cities and towns, often characterized by low-density residential housing, single-use zoning, and increased reliance on the private automobile for transportation. ". Urban sprawl has been described as the unrestricted growth in many urban areas of housing, commercial development, and roads over large expanses of land, with little concern for very dense urban planning. Sometimes the urban areas described as the most "sprawling" are the most densely populated. In addition to describing a special form of urbanization, the term also relates to the social and environmental consequences associated with this development. In modern times some suburban areas described as "sprawl" have less detached housing and higher density than the nearby core city. Medieval suburbs suffered from the loss of protection of city walls, before the advent of industrial warfare. Modern disadvantages and costs include increased travel time, transport costs, pollution, and destruction of the countryside. The revenue for building and maintaining urban infrastructure in these areas are gained mostly through property and sales taxes. Most jobs in the US are now located in suburbs generating much of the revenue, although a lack of growth will require higher tax rates.

In Europe, the term peri-urbanisation is often used to denote similar dynamics and phenomena, but the term urban sprawl is currently being used by the European Environment Agency. There is widespread disagreement about what constitutes sprawl and how to quantify it. For example, some commentators measure sprawl by residential density, using the average residential units per acre in a given area. Others associate it with decentralization (spread of population without a well-defined centre), discontinuity (leapfrogging development, as defined below), segregation of uses, and so forth.

The term urban sprawl is highly politicized and almost always has negative connotations. It is criticized for causing environmental degradation, intensifying segregation, and undermining the vitality of existing urban areas, and is attacked on aesthetic grounds. The pejorative meaning of the term means that few openly support urban sprawl as such. The term has become a rallying cry for managing urban growth.

The term urban sprawl was often used in the letters between Lewis Mumford and Frederic J. Osborn, firstly by Osborn in his 1941 letter to Mumford and later by Mumford, generally condemning the waste of agricultural land and landscape due to suburban expansions. The term was used in an article in The Times in 1955 as a negative comment on the state of London's outskirts. Definitions of sprawl vary; researchers in the field acknowledge that the term lacks precision. Batty et al. defined sprawl as "uncoordinated growth: the expansion of community without concern for its consequences, in short, unplanned, incremental urban growth which is often regarded unsustainable". Bhatta et al. wrote in 2010 that despite a dispute over the precise definition of sprawl, there is a "general consensus that urban sprawl is characterized by [an] unplanned and uneven pattern of growth, driven by a multitude of processes and leading to inefficient resource utilization".

Reid Ewing has shown that sprawl has typically been characterized as urban developments exhibiting at least one of the following characteristics: low-density or single-use development, strip development, scattered development, and/or leapfrog development (areas of development interspersed with vacant land). He argued that a better way to identify sprawl was to use indicators rather than characteristics because this was a more flexible and less arbitrary method. He proposed using "accessibility" and "lack of functional open space" as indicators. Ewing's approach has been criticized for assuming that sprawl is defined by negative characteristics.

What constitutes sprawl may be considered a matter of degree and will always be somewhat subjective under many definitions of the term. Ewing has also argued that suburban development does not, per se, constitute sprawl depending on the form it takes, although Gordon & Richardson have argued that the term is sometimes used synonymously with suburbanization in a pejorative way.

According to the National Resources Inventory (NRI), about 44 million acres (69,000 sq mi; 180,000 km²) of land in the United States was developed between 1982 and 2017. Presently, the NRI classifies approximately 100,000 more square kilometres (40,000 square miles) (an area approximately the size of Kentucky) as developed than the Census Bureau classifies as urban. The difference in the NRI classification is that it includes rural development, which by definition cannot be considered to be "urban" sprawl. Currently, according to the 2000 Census, approximately 2.6 percent of the U.S. land area is urban.^{[needs update]} Approximately 0.8 percent of the nation's land is in the 37 urbanized areas with more than 1,000,000 population. In 2002, these 37 urbanized areas supported around 40% of the total American population.^{[needs update]}

Nonetheless, some urban areas like Detroit have expanded geographically even while losing population. But it was not just urbanized areas in the U.S. that lost population and sprawled substantially. According to data in "Cities and Automobile Dependence" by Kenworthy and Laube (1999), urbanized area population losses occurred while there was an expansion of sprawl between 1970 and 1990 in Amsterdam, Netherlands; Brussels, Belgium; Copenhagen, Denmark; Frankfurt, Hamburg and Munich, Germany; and Zürich, Switzerland, albeit without the dismantling of infrastructure that occurred in the United States.^{[citation needed]}