Untouchability is a form of social institution that legitimises and enforces practices that are discriminatory, humiliating, exclusionary and exploitative against people belonging to certain social groups. Although comparable forms of discrimination are found all over the world, untouchability involving the caste system is largely unique to South Asia.

The term is most commonly associated with treatment of the Dalit communities in the Indian subcontinent who were considered "polluting". The term has also been used to refer to other groups, including the Burakumin of Japan, the Baekjeong of Korea, and the Ragyabpa of Tibet, as well as the Romani people and Cagot in Europe, and the Al-Akhdam in Yemen Traditionally, the groups characterized as untouchable were those whose occupations and habits of life involved ritually "polluting" activities, such as pursuing a career based on killing (e.g. fishermen) or engaging in common contact with others' feces or sweat (e.g. manual scavengers, sweepers and washermen).

According to the religious Hindu text, untouchables were not considered a part of the varna system. Therefore, they were not treated like the savarnas (Brahmins, Kshatriyas, Vaishyas and Shudras).

Due to many caste-based discriminations in Nepal, the government of Nepal legally abolished the caste-system and criminalized any caste-based discrimination, including "untouchability" in 1963.

Untouchability has been outlawed in India, Nepal and Pakistan. However, "untouchability" has not been legally defined. The origin of untouchability and its historicity are still debated. B. R. Ambedkar believed that untouchability has existed at least as far back as 400 AD. A recent study of a sample of households in India concludes that "Notwithstanding the likelihood of under-reporting of the practice of untouchability, 70 percent of the population reported not indulging in this practice. This is an encouraging sign."