Delhi (locally pronounced Dillee or Dehli), officially the National Capital Territory of Delhi (NCT), is the largest metropolis by area and the second-largest metropolis by population in India. It is the eighth largest metropolis in the world by population with 16.7 million inhabitants in the Territory at the 2011 Census. There are nearly 22.2 million residents in the greater National Capital Region urban area (which also includes the cities Noida, Greater Noida, Ghaziabad, Gurgaon and Faridabad along with other smaller nearby towns). The name Delhi is often also used to include urban areas near the NCT, as well as to refer to New Delhi, the capital of India, which lies within the metropolis. Although technically a federally administered union territory, the political administration of the NCT of Delhi today more closely resembles that of a state of India with its own legislature, high court and an executive council of ministers headed by a Chief Minister. New Delhi, jointly administered by both the federal Government of India and the local Government of Delhi, is also the capital of the NCT of Delhi.

It is a misconception that Delhi was awarded the status of "State" after The Constitution (69th Amendment) Act, 1991, which isn't the case. Delhi is still a union territory which has been given special status of National Capital Region. The Act made provision for Legislative assembly and a council of ministers for Delhi. Located on the banks of the River Yamuna, Delhi has been known to be continuously inhabited since at least the 6th century BCE, though human habitation is believed to have existed since the second millennium BCE. Delhi is also widely believed to have been the site of Indraprastha, the legendary capital of the Pandavas during the times of the Mahabharata. Delhi re-emerged as a major political, cultural and commercial city along the trade routes between northwest India and the Gangetic plain after the rise of the Delhi sultanates. It is the site of many ancient and medieval monuments, archaeological sites and remains. In 1639, Mughal emperor Shahjahan built a new walled city in Delhi which served as the capital of the Mughal Empire from 1649 to 1857.

After the British East India Company got a foothold in North East India in the late 18th century, Calcutta became the capital of British held territories under Company rule (1774–1857) and remained so under the British Raj (1857–1920). British had captured Delhi by 1803 and George V announced in 1911 that the capital of British controlled parts of India would move back to Delhi. A new capital city, New Delhi, was built to the south of the old city during the 1920s. When India gained independence from British rule in 1947, New Delhi was declared its capital and seat of government. As such, New Delhi houses important offices of the federal government, including the Parliament of India, as well as numerous national museums, monuments, and art galleries.

Owing to the migration of people from across the country (mostly from the Northern and Eastern states of India), Delhi has grown to be a multicultural, cosmopolitan metropolis. Its rapid development and urbanisation, coupled with the relatively high average income of its population, has transformed Delhi into a major cultural, political, and commercial centre of India. Delhi has plenty of parks and gardens. New Delhi, a part of Delhi is one of world's Greenest Capital cities. Delhi Ridge, the last leg of the Aravalli Range is known as Delhi's Green Lung. It runs through South Delhi and terminates in Central Delhi. This makes these two regions the greenest in the city. Owing to its greens, Delhi is home to such large number of birds that it is the world's most bird enriched Capital, only after Kenya's Nairobi.

The definition of capital of India New Delhi within Delhi is unclear and complex. Even administratively, the demarcation between Delhi and New Delhi is not clear. Government of India and Government of Delhi have not clearly defined the area and boundaries of New Delhi. The boundaries of areas of New Delhi Municipal Council (NDMC) and New Delhi District of Delhi do not completely overlap. Boundaries of New Delhi District of Delhi Police is again different from boundaries of NDMC and New Delhi district. For example, AIIMS falls within the NDMC boundary but outside the New Delhi districts of both Delhi Govt. and Delhi Police. It comes under South Delhi district of Delhi Govt. and Delhi Police. IIT Delhi falls under none of the New Delhi designated areas (i.e. NDMC and New Delhi district) but official address of IIT Delhi is IIT Delhi, Hauz Khas, New Delhi-110 016. Similarly, most of the government offices, institutes, post offices, fire stations and police stations located outside of the areas NDMC and New Delhi district have 'New Delhi' in their official addresses. In 1920s, when British constructed New Delhi, there were only two Delhis. Old walled city called Old Delhi and the new city designed by Edwin Lutyens to serve as capital of British India, which was called 'New Delhi'. Rest of Delhi was rural. But after independence as more and more areas around New Delhi urbanised they also became a part of New Delhi as they were also a part of Delhi which was 'new'. Now, almost all of National Capital Territory of Delhi, with exceptions of old city called Old Delhi, Civil Lines area, Trans-Yamuna region, some rural areas and some underdeveloped parts of Delhi (e.g. Ashok Vihar, Timarpur, Nangloi, Sultanpuri, Mangolpuri, Badarpur etc.), is designated as 'New Delhi' in postal addresses. Most of the times, the terms 'Delhi' and 'New Delhi' are used interchangeably.