Arcologies are defined by Wiktionary as: "Urban development theory proposed by Paolo Soleri involving three-dimensional building methods and efficient use of space and resources; An extremely large habitat or settlement, sufficient to maintain an internal ecology as well as an extremely high human population density." Wikipedia offers this definition: "Arcology, a portmanteau of the words 'architecture' and 'ecology,' is a set of architectural design principles aimed toward the design of enormous habitats (hyperstructures) of extremely high human population density." It should also be noted that further-on in the Wikipedia entry they amend the definition slightly with this restatement: "...An extremely large habitat or settlement, sufficient to maintain an internal ecology as well as an extremely high human population density."
Essentially an Arcology is a very large-scale, autonomous habitat-structure, sometimes referred to as a megacity or megastructure, that encloses both a human population and its own internal ecology, allowing these structures to occupy regions that otherwise might be completely uninhabitable such as on ocean floors (Deep Domes), suspended on tethers over gas giants, in the cold void of space, hitched to comets, nestled underneath the crustal shell of barren worlds, on the darkside of moons, or even less exotic locales such as the rims of the Great Rift or deep beneath the sunny streets of the Fifth Arrondissement as in the case of the Tenement Towers on Riskail or out amongst the Fringeworlds. At least one known Coreworld, Jezeal, is entirely enclosed within a single planetary-scale Arcological Complex (Arcplex) and it is rumored that there are variant forms of Concentriplexes enclosing entire planetary orbits amongst the Technophiliate, Autonocracy, Seimgress and other such Exocultures dedicated and oriented towards Macroscale and Megascale Engineering. The Gas Giant Archipelagos are another example of a form of megascale arcological development.
Whether or not the contained population is of any given size or type is no longer considered an important aspect of the root definition.
Paolo Soleri began construction on Arcosanti in 1970. Arcosanti is the first Arcology ever attempted along the lines established by Paolo Soleri in his various published works. He coined the terms 'Arcology,' 'Cosanti,' and 'Arcosanti.' Soleri is the secular saint of the arcology, the visionary who set things into motion and made the first one manifest as an example of what could be done, much as his former instructor Frank Lloyd Wright did with his Taliesin West, which is not too far away from Soleri's Arcosanti in Arizona.
This is something very real. The first steps have been taken and more is being learned and developed and experimented with each year as Arcosanti endures, evolves and inspires.
But two other visionaries had glimmerings of this concept before Soleri was able to make it his own. H. G. Wells described what might be one of the very first attempts to express the concept of an arcology in fiction in his novel When the Sleeper Wakes, and William Hope Hodgson described a fairly straight forward (for Hodgson) arcology with his megascale grey-metal Pyramid-shaped last redoubt in The Nightland. Others have since picked-up the concept of the arcology and made it all their own, not the least being Niven & Pournelle's city of Todos Santos featured in Oath of Fealty. There have been others, many, many others who have likewise made use of Soleri's brainchild, and why not? It's an absolutely fascinating idea and one that looks to be coming closer to manifest reality every decade past the Seventies we manage to get.
We haven't had the opportunity to read Ballard's High Rise yet, but it is on the list of books to get right away, and we've been looking over the Wikipedia entry for Paradise Towers from Doctor Who as recommended by Porky. What other instances of Arcologies should we investigate as possible sources of inspiration? Doesn't Warhammer 40K make use of Arcologies somewhat?