Short definition
Aging wine on its lees (dead yeast cells) to add richness, texture, and bread-like complexity.
After fermentation, dead yeast cells settle to the bottom. Instead of racking the wine off, some winemakers leave it sitting on these lees for months. The yeast slowly breaks down and releases compounds that make the wine richer, creamier, and more texturally interesting. Muscadet Sur Lie is the most famous still-wine example. All traditional-method sparkling wines age sur lie by definition.