buttermilk [buht-er milk] noun
The milky liquid left over after butter has been made fresh. Buttermilk you buy in the supermarket, however, is created by adding bacteria cultures to fat-free or low-fat milk.
Buttermilk is thicker than regular milk products, with a creamy mouthfeel and a slightly tangy flavor due to the light fermentation that occurs after the addition of the cultures. If you have never tasted this ingredient, think of it as a very thin, liquidy yogurt, but know that it is truly unique unto itself. Buttermilk is acidic, so it cannot be substituted for regular milk in baking without adjusting other components, such as leaveners.