Bihar is a state in eastern India. It is the third-largest state by population and twelfth-largest by territory, with an area of 94,163 km2 (36,357 sq mi). It is contiguous with Uttar Pradesh to its west, Nepal to the north, the northern part of West Bengal to the east, and with Jharkhand to the south. The Bihar plain is split by the river Ganges, which flows from west to east. Three main cultural regions converge in the state: Magadh, Mithila, and Bhojpur. Bihar is also the world's third-most populous subnational entity.

On 15 November 2000, southern Bihar was ceded to form the new state of Jharkhand. Only 11.3% of the population of Bihar lives in urban areas, which is the lowest in India after Himachal Pradesh. Additionally, almost 58% of Biharis are below the age of 25, giving Bihar the highest proportion of young people of any Indian state. The official languages are Hindi and Urdu, although other languages are common, including Maithili, Magahi, Bhojpuri and other Bihari languages.

At the 2011 Census, Bihar was the third most populous state of India with total population of 104,099,452, nearly 89% of it rural. It was also India's most densely populated state, with 1,106 persons per square kilometre. The sex ratio was 918 females per 1000 males. Almost 58% of Bihar's population was below 25 years age, which is the highest in India. At 11.3%, Bihar has the second-lowest urbanisation rate in India after Himachal Pradesh. As per 2011 Census, the Hinduism is practiced by 82.7% of the population and Islam was recorded as region of 16.9% to total population. Christianity (0.12%), Budhism (0.02%) and Sikhism (0.02%) is minority in religion in Bihar.