A capital city, or just capital, is the municipality holding primary status in a country, state, province, department, or other subnational division, usually as its seat of the government. A capital is typically a city that physically encompasses the government's offices and meeting places; the status as capital is often designated by its law or constitution. In some jurisdictions, including several countries, different branches of government are in different settlements, sometimes meaning multiple official capitals. In some cases, a distinction is made between the official (constitutional) capital and the seat of government, which is in another place.

English-language media often use the name of the capital metonymically to refer to the government sitting there. Thus, "London-Washington relations" is widely understood to mean diplomatic relations between Great Britain and the United States.

The word capital derives from the Latin word caput (genitive capitis), meaning 'head', later borrowed from Medieval Latin capitālis ('of the head'). The Latin phrase Roma Caput Mundi (lit. 'Rome head of the world') was already used by the poet Ovid in the 1st century BC. It originates out of a classical European understanding of the known world: Europe, North Africa, and Southwest Asia. The phrase is related to the enduring power of the city first as the capital of the Republic and the Empire, and later as the centre of the Catholic Church.

In several English-speaking states, the terms county town and county seat are also used in lower administrative divisions. In some unitary states, subnational capitals may be known as 'administrative centres'. The capital is often the largest city of its constituent, though not always.

Historically, the major economic centre of a state or region has often become the focal point of political power, and became a capital through conquest or federation. Historical examples are ancient Babylon, ancient Athens, ancient Rome, Abbasid Baghdad, Constantinople, Chang'an, and ancient Cusco. The modern capital city has not always existed: in medieval Western Europe, an itinerant (wandering) government was common.

The capital city attracts politically motivated people and those whose skills are needed for efficient administration of national or imperial governments, such as lawyers, political scientists, bankers, journalists, and public policy makers. Some of these cities are or were also religious centres, e.g. Constantinople (more than one religion), Rome/Vatican City (the Roman Catholic Church), Jerusalem (more than one religion), Babylon, Moscow (the Russian Orthodox Church), Belgrade (the Serbian Orthodox Church), Paris, and Beijing. In some countries, the capital has been changed for geopolitical reasons; Finland's first city, Turku, which had been the country's most important city since the Middle Ages and became capital in 1809, lost its position during the Grand Duchy of Finland in 1812, when Helsinki was made the current capital of Finland by the Russian Empire.

The convergence of political and economic or cultural power is by no means universal. Traditional capitals may be economically eclipsed by provincial rivals as is the case with Nanjing by Shanghai, Quebec City by Montreal, and several US state capitals. The decline of a dynasty or culture could also mean the extinction of its capital city, as occurred at Babylon and Cahokia. "Political nomadism" was practiced in ancient Near East to increase ties between the ruler and the subjects.

Although many capitals are defined by constitution or legislation, many long-time capitals have no such legal designation, including Bern, Edinburgh, Lisbon, London, Paris, and Wellington.^{[citation needed]} They are recognized as capitals as a matter of convention, and because all or almost all the country's central political institutions, such as government departments, supreme court, legislature, embassies, etc., are located in or near them.