City and Union Territory, north-central India. Popularly known as Old Delhi, it is the third largest city of India, surpassed in population only by Calcutta and Greater Bombay. New Delhi, the capital of the Indian Union, lies immediately to the south. Besides being at the political centre of the country, Delhi is also a focal point in India's transportation network.

Delhi is situated about 100 miles (160 kilometres) south of the Himalayas and stands on the west (right) bank of the Yamuna River, a tributary of the Ganges. The union territory, which lies at an altitude of between 700 and 1,000 feet (213 and 305 metres), covers an area of 573 square miles (1,485 square kilometres). Of this area, Old Delhi occupies 360 square miles and New Delhi 169 square miles. The union territory is bounded on the east by the state of Uttar Pradesh and on the north, west, and south by Haryana. It generally has been presumed that the city was named for Raja Dhilu, a king who reigned in the 1st century BC , and that the various names by which it has been known (Delhi, Dehli, Dilly, and Dhilly) have been corruptions of this name.

Delhi has been the capital city of a succession of mighty empires and powerful kingdoms, and numerous ruins mark the sites of the various cities. According to popular tradition, the city has changed its locality a total of seven times, although some authorities, who take smaller towns and strongholds into account, claim it has changed its site as many as 15 times. All these locations are confined to a triangular area of about 70 square miles called the Delhi triangle. Two sides of this triangle are represented by the rocky hills of the Ar(valli Range in the west and south and the third side by the shifting channel of the Yamuna River. The present site of Delhi is bounded to the west by a northern extension of the Ar(valli Range known as the Delhi Ridge.

The name "Delhi" is of uncertain etymology. One suggestion is that its eponym is "Dhillu", the name of a king who ruled the area. However, some historians believe that the word Dilli, another name for Delhi, originated from the Persian word dahleez, meaning "frontier" or "threshold". Another theory suggests that the city's original name was Dhillika. The Persianized surname Dahelvi is also related to residents of Delhi. The Hindi/Prakrit word dhili ("loose") was also used for the locality, gradually morphing into the local name "Dilli".