A city-state is a region controlled exclusively by a city, usually having sovereignty. Historically, city-states have often been contingent of larger cultural areas, as in the city-states of ancient Greece such as (Athens, Sparta, Thebes, and Corinth), the Phoenician cities of Canaan (such as Tyre and Sidon), the Mayans of pre-Columbian Mesoamerica (including sites such as Chichen Itza and El Mirador), the central Asian cities along the Silk Road (which includes Samarkand and Bukhara), or the city-states of Northern Italy (especially Florence and Venice). More recently the neologism citistate has been developed, referring to the city as the center of a 'city region' including relocated urban business like factory and company towns, and supply economies like agricultural, timber and mineral extraction whose commodities find a market in the city. The term "city-state" should not be confused with "independent city", which refers to a city which is not administered as part of another local government area (eg, a county).

Among the most well known periods of city-state culture in human history include ancient Greek city-states, and the merchant city-states of Renaissance Italy, who organized themselves in small independent centers. The success of small regional units coexisting as autonomous actors in loose geographical and cultural unity, as in Italy or Greece, often prevented their amalgamation into larger national units. However, such small political entities often survived only for short periods because they lacked the resources to defend themselves against incursions by larger states. Thus they inevitably gave way to larger organizations of society, including the empire and the nation-state.

Whereas the nation-states rely on a common cultural heritage, be it linguistic, historical, religious, economic, etc., the city-state relies on the common interest in the function of the urban center. The urban center and its activity supplies the livelihoods of all urbanites inhabiting the city-state.

Several sovereign countries do have self-governing areas delineated around cities, but these are not true city-states because they are not independent of the larger state. Typically a federal country will have a federal administration from which the capital is granted a separate status. Examples include Washington, D.C. in the United States, Brasília (coterminous with the Brazilian Federal District) in Brazil, Mexico City (being the Mexican Federal District), Canberra (part of the Australian Capital Territory) and Ciudad Autónoma de Buenos Aires (coterminous with the Argentine Federal Capital).

In nations without a federal administrative structure, capital cities sometimes enjoy a greater degree of autonomy, e.g. London. In Germany, Berlin has been a federal state (Land) of its own since 1990, but, unlike the above, remains inside the general administration of the country. (Note but this has nothing to do with its being the capital; the cities of Hamburg and Bremen have an identical status and look back on a much longer history of political independence.) A similar situation can be found in Austria, where the capital Vienna has the status of a state within the Austrian federation of states.