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This is Ritika

 Untouchability is the quality or condition of being an untouchable, ascribed in the Vedic tradition to persons of low caste or to persons excluded from the caste system.

In the ancient time low caste people was not allowed to seat in same level with upper caste, children from low caste not allowed to seat in bench, woman cannot get water from well ...etc.

Basically it is the caste discrimination and low caste people are known as untouched or untouchable and the practice through which they go through is untouchability.

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