Buttermilk

Buttermilk was once used to refer to the liquid left over after making butter from cultured cream. In this procedure, milk was left out to rest for a period of time. It allows the cream and milk to separate, leaving behind the thick cream top that is needed to make butter. Buttermilk was produced as a result of the milk sugars being fermented by naturally occurring lactic acid bacteria during the resting phase.

It's still widely used in Pakistan and India, but less so in the West. Commercial buttermilk is pasteurized, much like sour cream, and the bacteria are added after the heating process. Although it has a tangy flavor that is similar to sour cream, it may only be substituted in baked goods or dressings.