ullage

vaje In a wine bottle, the empty space between the cork and the top of the wine.
In a cask or barrel, the empty space, occupied by air, that is created by not completely filling the cask or barrel
The topping-up of such a barrel with fresh wine
In an industrial setting, the empty space in a tank, such as for fuel
To gauge the amount of empty space between the top of a cask and the level of liquid inside it.