Delhi, officially the National Capital Territory of Delhi, is a city and a union territory of India. It is bordered by Haryana on three sides and by Uttar Pradesh to the east. The NCT covers an area of 1,484 square kilometres. According to the 2011 census, Delhi city's proper population was over 11 million, the second highest in India after Mumbai, while the whole NCT's population was about 16.8 million. Delhi's urban area is now considered to extend beyond the NCT boundary to include an estimated population of over 26 million people, making it the world's second largest urban area. As of 2016 recent estimates of the metro economy of its urban area have ranked Delhi either the top or second most productive metro area of India. Delhi is the second wealthiest city after Mumbai in India, with a total wealth of $450 billion and home to 18 billionaires and 23,000 millionaires.
Delhi has been continuously inhabited since the 6th century BC. Through most of its history, Delhi has served as a capital of various kingdoms and empires.