The city where a country or region has its government.
Capital is a noun that refers to a municipality in a country or state where the government operates. The capital city of a country is generally the economic and cultural centre of a nation, for example Brussels, London and Tokyo. Etymologically, the word derives from caput in Latin, meaning ‘head’.
Major capital cities in history have arisen as the dominant economic centres of different states, which has led to the metropolis becoming the focal point for political power. Most capitals are also the largest cities in terms of population and size in their respective countries. Historically, the capital is often the main target during wartime, as the capture of it in a siege means the collapse of a rival government.
capital or capital city [COUNTABLE] the city where a country or region has its government
Madrid is the capital of Spain.
state/regional/provincial capital: Tallahassee is the state capital of Florida.
You can see the full definition in Macmillan Dictionary here.