ullage

About 0.03 uses per million words.

Meaning

noun In a wine bottle, the empty space between the cork and the top of the wine.
noun In a cask or barrel, the empty space, occupied by air, that is created by not completely filling the cask or barrel, or through spillage.
noun The topping up of such a barrel with fresh wine.
noun In an industrial setting, the empty space in a tank, especially as for fuel.
noun Additional cargo of little or no value taken on to prevent movement or shifting of the purposive cargo.
noun Leftover wine surreptitiously drunk by waiters as they clear away the glasses.
verb To gauge the amount of empty space between the top of a cask and the level of liquid inside it.