首都 means "capital city" and refers to the city that serves as the seat of government for a country. It combines 首 (head/chief) with 都 (capital/metropolis). It is used in both formal and everyday contexts, as in 北京是中国的首都 (Beijing is China's capital).
北京是中国的首都,历史非常悠久。
Běijīng shì Zhōngguó de shǒudū, lìshǐ fēicháng yōujiǔ.
Beijing is the capital of China and has a very long history.
你知道澳大利亚的首都是哪座城市吗?
Nǐ zhīdào Àodàlìyà de shǒudū shì nǎ zuò chéngshì ma?
Do you know which city is the capital of Australia?
每个国家的首都都有它独特的魅力。
Měi ge guójiā de shǒudū dōu yǒu tā dútè de mèilì.
Every country's capital has its own unique charm.