Hong Kong, officially the Hong Kong Special Administrative Region of the People's Republic of China, is an autonomous territory on the eastern side of the Pearl River estuary in East Asia. Along with Macau, Guangzhou, Shenzhen, Zhuhai, and several other major cities in Guangdong, the territory forms a core part of the Pearl River Delta metropolitan region, the most populated area in the world. With over 7.4 million Hongkongers of various nationalities[note 1] in a territory of 1,104 square kilometres (426 sq mi), Hong Kong is the fourth-most densely populated region in the world.
Hong Kong was formerly a colony of the British Empire, after the perpetual cession of Hong Kong Island from Qing China at the conclusion of the First Opium War in 1842. Originally a lightly populated area of farming and fishing villages, the territory has become one of the most significant financial centres and trade ports in the world. With the exception of the Second World War, during which the colony was occupied by the Empire of Japan, Hong Kong remained under British control until 1997, when it was returned to China. As a special administrative region, Hong Kong maintains a separate political and economic system apart from mainland China.
As the world's seventh-largest trading entity, its legal tender, the Hong Kong dollar, is the 13th-most traded currency. Hong Kong's tertiary sector dominated economy is characterised by competitive simple taxation and supported by its common law judiciary system. Although the city boasts one of the highest per capita incomes in the world, it suffers from severe income inequality.
The territory features the most skyscrapers in the world, surrounding Victoria Harbour, which lies in the centre of the city's dense urban region. It has a very high Human Development Index ranking and the seventh-highest life expectancy in the world. Over 90% of its population makes use of well-developed public transportation. Seasonal air pollution originating from neighbouring industrial areas of mainland China, which adopts loose emissions standards, has resulted in a high level of atmospheric particulates in winter.