Buttermilk

Chaas is a curd-based drink popular across the Indian subcontinent. In Magahi (Magahi Language) and Bundeli, it is called Mattha. In Rajasthani it is called ghol, in Odia it is called Ghol/Chaash, moru in Tamil and Malayalam, taak in Marathi, majjiga in Telugu, majjige in Kannada, ale (pronounced a-lay) in Tulu and ghol in Bengali. In Indian English, it is often referred to as buttermilk. The name Chaas or Chaach is derived from Sanskrit word Chacchika (छच्छिका), meaning churned yogurt from which butter has been removed. Chaas is made by churning yogurt (curds/dahi) and cold water together in a pot, using a hand-held instrument called madhani (whipper). This can be consumed plain or seasoned with a variety of spices or made sweet (and then known as Lassi). Chaas can be made from fresh yogurt, and the natural flavour of such chaas is mildly sweet.

Source: Wikipedia