An arcology is a term used to describe a large-scale human habitat, pre-planned, and sometimes of one contiguous structure. The word "arcology" itself is a portmanteau of "architectural ecology," coined by Paolo Soleri,  although the popular meaning of the term has broadened to cover several similar large-scale human habitats seen in fiction. However, they are not limited to the realm of science fiction, as several governments and private businesses are actively researching arcologies to put them into practical use as soon as 2010.
Galen Cranz, of the Illinois Institute of Technology, writes this description in reviewing Paolo Soleri's book:
When built on a city scale, its [an arcology’s] density frees surrounding countryside to host natural ecological cycles, thereby restoring the earth, the air, and plant and animal life to an unpolluted state. Like the medieval city, the arcology allows the citizen to walk beyond its limits to the country; thus, man would become both a city and country dweller. Internally, the arcology (city) is free from pollution because cars are not necessary. Industry, housed in the inner core of these giant structures, produces by-products such as heat and wind, which in specific arcologies, Soleri proposes, would heat the remainder of the environment, thereby eliminating wasteful duplication of energy production. 
Much like any planned city, an arcology can be built with an end result in mind, reducing costs. However, unlike most planned cities, arcologies are usual multi-story affairs, reducing travel time by adding vertical distance -- effectively dividing the distance needed to travel by the number of stories in the town. Because of this, an arcology using walkways instead of roads as the major source of travel would save. As an example An American arcology using walkways in lieu of would save thousands of dollars per person per year, not only in car maintenance fees but in gasoline and in necessary infrastructure to
There are other benefits as well. By reducing the distance between any two points in the city, public safety can be increased -- the response time for emergency services, whether police, medical, or fire, would be reduced by living in an arcology.
Additionally, arcologies do not to be singular, monolithic structures, as commonly seen in fiction; most arcologies being designed today are scalable. These smaller arcologies are designed to support a small number, but can link together by linking smaller related arcologies, creating a 'compartamentalized' arcology. A single-building arcology could support the population of a small town (5,000), and additional five-thousand-person arcologies could be built later in neighboring lots, effectively creating a ten-thousand, fifteen-thousand, or larger arcology.
Although arcologies have notable advantages, there are many limitations inherent in their design. For example, unless notable advantages in hydroponics are made, no arcology could grow all its own food -- and even then, foods grown outside arcologies would likely be cheaper, even with the cost of transport added in. For the time being, although an arcology could be built today, they cannot be self-sufficient.
Arcologies, like some fallout shelters, could face problems with overheating, with more heat being generated by electricity faster than the reduced surface area of the arcology could dissipate it. Problems with venting waste heat could prevent arcologies from being built in hot climates, or could severely impact their design.
Outbreaks in an arcology would spread faster, as both humans and habitations are closer together. What would be a minor fire could burn an arcology to the ground literally overnight, and a viral outbreak would devastate a community, especially in nations without central health care.
In addition, some economists think arcologies would reduce competition. For example, if a Wal-Mart was in an arcology, it would be within walking distance of a greater number of people than a Wal-Mart in a conventional town, making it harder for smaller businesses to compete, stunting economic growth.
Several designs for arcologies are designed to be built on sea, not land. These designs can be broken down into three broad categories:
- Artificial islands, essentially using the same techniques that are used to design land-based arcologies, but instead building over the ocean. One advantage of the artificial island design is that it can allow for the same economic benefits as a port.
- City-ships, giant ships that can sustain entire populations. This was inspired by the fact that several large ships (including tankers and nuclear carriers) are already equipped to spend months or years at sea, and (theoretically) could be built for increased or indefinite sustainability for populations.
- Subaquatic arcologies (or "sea stations"), which work similar to space stations but are designed to remain underwater.
One aquatic arcology design, Freedom Ship, has attracted relatively high amounts of media attention and criticism. Its design, similar to a barge's but at a much larger scale, would support a permanent population of about 25'000, including schools, a casino, a mall, an airstrip, and 100 acres of open 'land' (parks and recreation areas). The completeness of the plans has attracted investors, although not enough to begin construction.
Arcologies in fiction
Arcologies have become popular in fiction, being used in utopias and dystopias alike.
- Some aquatic arcologies have featured heavily in fiction, such as seaQuest DSV and Sealab 2020 (and the series that parodied it, Sealab 2021.
- The video games Deep Sea Tycoon (and its sequel both set you in the position of running an subaquatic arcology.
- Many cyberpunk games and novels feature arcologies in one form or another, used to represent a "high-tech" variation of the housing projects which famously failed in the 1980s, the same decade that the cyberpunk literary movement took hold.
- Sim City offers arcologies in many versions of the game, as a high-density form of housing.
- Paolo Soleri. (1969) Arcology: The City in the Image of Man. MIT Press, Cambridge, Maryland.
- Paolo Soleri. (2006) Arcology: The City in the Image of Man. Cosanti Press. ISBN 9781883340018. | Oversized format, "outstanding graphics.
- Galen Cranz. (1971) Review of: Arcology: The City in the Image of Man. by Paolo Soleri. American Journal of Sociology 77(2):377-379.