Definition - What does Ullage mean?
Ullage refers to the air space between wine and the top of the barrel or bottle. It also refers to the evaporation process that leads to the air space.
The amount of ullage is indicative of the storage conditions and quality of the wine. Excessive ullage is a tell-tale sign that a wine has undergone harmful oxidation.
WineFrog explains Ullage
Ullage in the wine barrel most commonly occurs due to evaporation. Vapor particles can leak out through spaces in the wood or around the stopper. Too much contact with oxygen will spoil the wine. In order to keep this oxidation at bay, winemakers create "topping schedules." At a predetermined time interval, the winemaker will add more wine taken from a smaller container to top off the barrels, preventing excessive ullage.
Also known as the fill level, ullage in a bottle of wine refers to the space between the wine and the bottom of the cork. Corks are not air tight, so some evaporation is to be expected. The more ullage a bottle shows, the more likely a wine has suffered oxidation and spoilage, resulting in the loss of fruit or brownish coloring of the wine. Ideally, no more than a half inch to an inch of air space should be visible between the wine and the cork, depending upon the age of the wine.