Barm, a wor derived from the old English for yeast, is the foam, or scum, formed on the top of liquor when fermenting. It was used to leaven bread, or set up fermentation in a new batch of liquor. Barm, as a leaven, has also been made from ground millet combined with must out of wine-tubs and is sometimes used in British baking as a synonym for a natural leaven.