Ullage
WordNet
noun
(1) The amount that a container (as a wine bottle or tank) lacks of being full
WiktionaryText
Noun
- 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
- In an industrial setting, the empty space in a tank, such as for fuel