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Untouchability is a social practice within the caste system in which members of the lowest castes are thought to be ritually impure to such a limit that they cause pollution by mere touch or even sight. Untouchable castes are at the bottom of the hierarchical system. These low castes are excluded from most social institutions.
It is an extreme and vicious aspect of the caste system. It uses stringent social and ritual sanctions against members of castes located at the bottom of the purity pollution scale. They are considered to be outside the caste hierarchy.