Macau (/məˈkaʊ/; traditional Chinese: 澳門; simplified Chinese: 澳门; Jyutping: ou3mun4*2; pinyin: Àomén), also spelled Macao, is one of the two Special Administrative Regions of the People's Republic of China, the other being Hong Kong. Macau lies on the western side of the Pearl River Delta across from Hong Kong, which is about 64 kilometers to the east, and it is also bordered by Guangdong Province to the north and the South China Sea to the east and south. With an estimated population of around 624,000 living in an area of 31.3 km2 (12.1 sq mi), it is the most densely populated region in the world.
A former Portuguese colony, Macau was administered by Portugal from the mid-16th century until late 1999, when it was the last remaining European colony in Asia. Portuguese traders first settled in Macau in the 1550s. In 1557, Macau was rented to Portugal by the Ming Dynasty as a trading port. The Portuguese administered the city under Chinese authority and sovereignty until 1887, when Macau became a colony of the Portuguese empire. Sovereignty over Macau was transferred back to China on 20 December 1999. The Sino-Portuguese Joint Declaration and the Basic Law of Macau stipulate that Macau operate with a high degree of autonomy until at least 2049, fifty years after the transfer.
Under the policy of "one country, two systems", the PRC's Central People's Government is responsible for the territory's defense and foreign affairs, while Macau maintains its own legal system, police force, monetary system, customs policy, and immigration policy. Macau participates in many international organizations and events that do not require members to possess national sovereignty.
Macau is one of the world's richest cities, with the highest GDP per capita by purchasing power parity as of 2013, according to the World Bank. It became the world's largest gambling centre in 2006, with the economy heavily dependent on gambling and tourism, as well as manufacturing. Cantonese people from Hong Kong and Guangdong, in addition to the recent mainland tourism from Mandarin-speaking regions, have boosted the economy of Macau significantly. According to The World Factbook, Macau has the second highest life expectancy in the world. Moreover, it is one of only a few regions in Asia with a "very high Human Development Index", ranking 25th as of 2011.