Delhi, officially the National Capital Territory of Delhi (NCT), is a city and a union territory of India containing New Delhi, the capital of India. It is bordered by the state of 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's city 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 boundaries, and include the neighbouring satellite cities of Ghaziabad, Faridabad, Gurgaon and Noida in an area now called National Capital Region (NCR) and had an estimated 2016 population of over 26 million people, making it the world's second-largest urban area according to the United Nations. As of 2016, recent estimates of the metro economy of its urban area have ranked Delhi either the most or second-most productive metro area of India. Delhi is the second-wealthiest city in India after Mumbai and is home to 18 billionaires and 23,000 millionaires. Delhi ranks fifth among the Indian states and union territories in human development index. Delhi has the second-highest GDP per capita in India.

Furthermore, it is considered one of the world's most polluted cities by particulate matter concentration.

Delhi has been continuously inhabited since the 6th century BCE. Through most of its history, Delhi has served as a capital of various kingdoms and empires, most notably the Delhi Sultanate and the Mughal Empire. The city has been captured, ransacked and rebuilt several times, particularly during the medieval period, and modern Delhi is a cluster of a number of cities spread across the metropolitan region.