The nilgai (Boselaphus tragocamelus), sometimes called nilgau, is an antelope, and is one of the most commonly seen wild animals of central and northern India and eastern Pakistan; it is also present in parts of southern Nepal. The species has become extinct in Bangladesh. The mature males appear ox-like and are also known as blue bulls. The nilgai is the biggest Asian antelope.

A blue bull is called a nil gai or nilgai in India, from nil meaning blue and gai meaning a bovine animal (literally 'cow'). In fact nilgai were known as the Nilghor (nil = blue, ghor = horse) during the rule of Aurangzeb (Mughal Era).

Nilgai stand 1.1 to 1.4 metres (3 ft 7 in to 4 ft 7 in) at the shoulder and measure 1.7 to 2.1 metres (5 ft 7 in to 6 ft 10 in) in head-body length, with a 45 to 50 centimetres (18 to 20 in) tail. Males are larger than females, weighing around 240 kilograms (530 lb), compared with the adult female weight of around 110 kilograms (240 lb).

Nilgai have thin legs and a robust body that slopes down from the shoulder. They show marked sexual dimorphism, with only the males having horns. Adult males have a grey to bluish-grey coat, with white spots on the cheeks and white colouring on the edges of the lips. They also have a white throat bib and a narrow white stripe along the underside of the body that widens at the rear. The tips of the long tufted tail and of the ears are black. They also possess a tubular shaped "pennant" of long, coarse, hair on the midsection of the throat.