ullage [UHL-ihj]

from

The empty space that develops in bottles, barrels, or casks as wine evaporates. It's important for the ullage in casks or barrels to be kept to a minimum by TOPPING so that the air exposure won't cause OXIDATION. Older bottles of wine may have a larger space between the CORK and the wine owing to slow leakage or evaporation over time. However, a young bottle of wine with a large ullage could indicate a faulty cork.