Aadhaar is a 12-digit unique identity number

Aadhaar (English: Foundation) is a 12-digit unique identity number issued to all Indian residents based on their biometric and demographic data. The data is collected by the Unique Identification Authority of India (UIDAI), a statutory authority established in January 2009 by the Government of India, under the jurisdiction of the Ministry of Electronics and Information Technology, following the provisions of the Aadhaar (Targeted Delivery of Financial and other Subsidies, benefits and services) Act, 2016.

Aadhaar is the world's largest biometric ID system, with over 1.19 billion enrolled members as of 30 November 2017, representing over 99% of Indians . World Bank Chief Economist Paul Romer described Aadhaar as "the most sophisticated ID programme in the world". Considered a proof of residence and not a proof of citizenship, Aadhaar does not itself grant any rights to domicile in India. In June 2017 the Home Ministry clarified that Aadhaar is not a valid identification document for Indians travelling to Nepal and Bhutan. Aadhaar has been compared to the Social Security number of the United States, although Aadhaar has more uses and fewer safeguards.

Prior to the enactment of the Act, the UIDAI functioned, since 28 January 2009, as an attached office of the Planning Commission (now NITI Aayog). On 3 March 2016 a money bill was introduced in the Parliament to give legislative backing to Aadhaar. On 11 March 2016 the Aadhaar (Targeted Delivery of Financial and other Subsidies, benefits and services) Act, 2016, was passed in the Lok Sabha.