Lees

Lees are the remnants of yeast cells that accumulate and settle to the bottom of the container when fermentation is complete.

Stirring the lees (the cloudy sediment seen in the barrel.) Photo from kysela.com

After fermentation, the wine is usually poured off from its lees. However, full-bodied white wines may be left in contact with the lees (called sur lie) to add a richer texture and complexity to the wine.

In Burgundy, the lees are regularly stirred (a process called bâtonnage) during this contact period to further the effect of sur lie.

(The Wine Bible, Karen MacNeil.)