The word 'Dalit' is used to identify the roughly 250 million people in India who are found at the bottom of the Indian social structure called the caste system and are otherwise also commonly known as 'untouchables'.
The word 'Dalit' is of recent coinage and came into use from the 1970s onwards when the Dalit Panthers, a revolutionary group approriated this word to idenity themselves with dignity as opposed to the derogative names that are traditionally used to identify them.
The word 'Dalit' itself is derived from the sanskrit word 'Dal' which means ground, crushed, decimated - a condition in which the Dalits find themselves ever since the dawn of the caste system in India.
Dalits are found in every nook and corner of India and they are the ones who perform what is known as the Indian nation's demeaning work and jobs such as cleaning of drains and gutters, disposal of dead carcasses and bodies, cleaning of toilets and sweeping the streets. among other similar jobs.
Dalits usually live in secluded colonies both in urban as well as rural areas. As per the last census records they comprise 30% of the nations population.
This, of course, is a theoritical definition of the word 'Dalit' but the true definition of what it means to be a Dalit is only known to a Dalit for it is only he who experiences what it is to be a outcaste a untouchable on a daily basis all his life.
Note: This section provides very scanty information of the meaning of the word 'Dalit'. What it means to BE a Dalit is quite something else. You can get a detailed idea about Dalits in the eBook 'Truth About Dalits.