A barm is the foam or scum formed on the top of a fermenting liquid such as beer, wine, or other malt liquors. It is essentially a natural yeast formed during fermentation and has traditionally been used as a leavening agent in bread making or to start fermentation in new batches of liquor. Barm is sometimes called ale yeast and is a precursor to many forms of brewer's and baker's yeast used today. Additionally, in certain parts of Northern England, especially around Manchester, a "barm cake" refers to a soft, round, flattish bread roll traditionally leavened with barm. This bakery item is often used for sandwiches, including popular fillings like bacon or chips, and is regionally distinct from other types of bread rolls known by different names in other parts of the UK. In summary:

• Barm: yeast foam from fermenting malt liquors used for leavening bread or fermenting beverages.
• Barm cake: a regional type of soft bread roll in North West England traditionally made using barm.

This tradition highlights the role of natural fermentation in baking and brewing before commercial yeast became widely available. Barm can take longer to rise than commercial yeast but imparts a unique yeasty flavor to bread and baked goods. So, "barm" can refer to both the fermenting yeast substance and to a bread roll leavened with that substance in specific regional contexts.