Vat/væt/ Definition 1 of 3Pro Tip 1/3Industrial Scale The term 'vat' often refers to large-scale operations, not personal use. The brewery added hops to the vat for fermentation. Pro Tip 2/3Storage or Processing 'Vat' implies a container for storing bulk liquids or for processing, like in winemaking. Fermentation vats are critical in winemaking. Pro Tip 3/3Not Always Open Vats can be open-topped for fermentation or closed for storage. The vats in the factory were sealed to prevent contamination.