A ghost town, deserted city, extinct town, or abandoned city is an abandoned settlement, usually one that contains substantial visible remaining buildings and infrastructure such as roads. A town often becomes a ghost town because the economic activity that supported it (usually industrial or agricultural) has failed or ended for any reason (e.g. a host ore deposit exhausted by mining). The town may have also declined because of natural or human-caused disasters such as floods, prolonged droughts, extreme heat or extreme cold, government actions, uncontrolled lawlessness, war, pollution, or nuclear and radiation-related accidents and incidents. The term can sometimes refer to cities, towns, and neighborhoods that, though still populated, are significantly less so than in past years; for example, those affected by high levels of unemployment and dereliction.

Some ghost towns, especially those that preserve period-specific architecture, have become tourist attractions. Some examples are Bannack, Montana and Oatman, Arizona in the United States; Barkerville, British Columbia in Canada; Craco and Pompeii in Italy; Aghdam in Azerbaijan; Kolmanskop in Namibia; Pripyat and Chernobyl in Ukraine; Dhanushkodi in India; Fordlândia in Brazil and Villa Epecuén in Argentina.

T. Lindsey Baker, author of Ghost Towns of Texas, defines a ghost town as "a town for which the reason for being no longer exists." Some writers discount settlements that were abandoned as a result of a natural or human-made disaster or other causes; they restrict the term to settlements that were deserted because they were no longer economically viable. Some believe that any settlement with visible tangible remains should not be called a ghost town; others say, conversely, that a ghost town should contain the tangible remains of buildings. Whether or not the settlement must be completely deserted, or may contain a small population, is also a matter for debate. Generally, though, the term is used in a looser sense, encompassing any and all of these definitions. American author Lambert Florin defined a ghost town as "a shadowy semblance of a former self."

Factors leading to the abandonment of towns include depleted natural resources, economic activity shifting elsewhere, railroads and roads bypassing or no longer accessing the town, human intervention, disasters, massacres, wars, the shifting of politics or fall of empires, and volcanic eruptions. A town can also be abandoned when it is part of an exclusion zone due to natural or human-made causes.

Ghost towns may result when the single activity or resource that created a boomtown (e.g., nearby mine, mill or resort) is depleted or the resource economy undergoes a "bust" (e.g., catastrophic resource price collapse). A gold rush often brought intensive but short-lived economic activity to a remote village, only to leave a ghost town once the resource was depleted.

Boomtowns can often decrease in size as quickly as they grew. Sometimes, all, or nearly all, of the population can desert the town, resulting in a ghost town. The dismantling of a boomtown can often occur on a planned basis. Mining companies nowadays will create a temporary company town to service a mine site, building all the accommodations, shops and services required, and then remove them once the resource has been extracted. Modular buildings can be used to facilitate the process.

In some cases, multiple factors may remove the economic basis for a community; some former mining towns on U.S. Route 66 suffered both mine closures when the resources were depleted and loss of highway traffic as US 66 was diverted from places like Oatman, Arizona, onto a more direct path. Mine and pulp mill closures have led to many ghost towns in British Columbia, Canada, including several relatively recent ones: Ocean Falls, which closed in 1973 after the pulp mill was decommissioned; Kitsault, whose molybdenum mine shut down after only 18 months in 1982; and Cassiar, whose asbestos mine operated from 1952 to 1992.

In other cases, the reason for abandonment can arise from a town's intended economic function shifting to another, nearby place. This happened to Collingwood, Queensland, in Outback Australia when nearby Winton outperformed Collingwood as a regional centre for the livestock-raising industry. The railway reached Winton in 1899, linking it with the rest of Queensland, and Collingwood was a ghost town by the following year. More broadly across Australia, there has been a shift towards fly-in fly-out arrangements over building a company town, in order to avoid the development of ghost towns once a mining resource has been fully extracted.