buttermilk
In plain and simple terms buttermilk is the residual sour liquid left after the churning of butter. The liquid fermented overnight. It is great to use to make biscuits.
It tastes a bit like yogurt, and its consistency is thick. My husband tells me that his grandfather used to like to drink it straight. I cannot even imagine.
Actually he was probably ahead of his time in digestive health issues without even knowing it. Buttermilk is high in probiotics which aid in digestion. However, it should be unpasteurized since the process kill off all the good bacteria.
Some people like to use buttermilk when they are making fried chicken or fish because it creates a thick batter. Also when it reacts with the flour, it makes for a nice crust.