What is the difference between Delhi and New Delhi?

Delhi can refer to the Union Territory of Delhi NCT; the city of Delhi; and the National Capital Region of Delhi, which includes Delhi NCT as well as some areas from nearby states. It can even refer to Delhi Cantonment (which, together with the cities of Delhi and New Delhi, make up Delhi NCT).

On the other hand, New Delhi is a planned city in the UT of Delhi, adjacent to the city of Delhi, and is the seat of government of the UT of Delhi as well as the capital (seat of government) of India. The city was planned and constructed in the first half of 20th century so that the British could shift the capital of India from Calcutta to the more central Delhi. New Delhi is adjacent to Old Delhi (known as Delhi; see above), the historic capital of the Mughal Empire (constructed by Shah Jahan).