As yeast cells feed on the fermentable carbohydrates in the wort, the specific gravity of the liquid drops. This process thins the body of the beverage and increases its alcohol by volume. A brew that fails to do this fully will leave behind residual sweetness and a heavy mouthfeel, which can lead to off-flavors or packaging instability.
The stout didn't attenuate as much as I wanted, so we're stuck at a final gravity of ten twenty and it tastes a bit cloying.
Related terms
Attenuation
The percentage of original sugars in a wort or must that yeast has converted into ethanol and carbon dioxide.
Attenuative
Describing a yeast strain's inherent capability to ferment a high proportion of available sugars.
Yeast-bitten
Describing a beer that has taken on harsh, bitter, or meaty off-flavors from prolonged contact with dead yeast.