A layer of deposits of dead yeast or residual yeast and other particles that precipitate, or are carried by the action of “fining”, to the bottom of a vat of wine after fermentation and aging.  “Gross lees” refers to a thick layer of grape skins, seeds, and dead yeast that fall out of wine following pressing;  “fine lees” are the residual dead yeast and other particles that continue to fall out of wine over time due to gravity or as a result of the addition of fining agents, and is typically not as thick as the gross lees layer.  Wine is racked off of the lees in order to avoid the development of “off-tastes” as well as to increase clarification.