Short definition
Dead yeast cells and grape particles that settle to the bottom of a tank or barrel after fermentation.
Lees look like muddy sediment. Gross lees (chunky grape bits) are usually removed quickly. Fine lees (spent yeast) can be left in contact with the wine for months. The winemaker sometimes stirs them — called bâtonnage — to increase richness. Lees contact adds body, creaminess, and brioche-like flavors. It also protects wine from oxidation, reducing the need for added sulfites.