What is the meaning of Ullage?

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, or through spillage.

The topping up of such a barrel with fresh wine.

In an industrial setting, the empty space in a tank, especially as for fuel.

Additional cargo of little or no value taken on to prevent movement or shifting of the purposive cargo.

Leftover wine surreptitiously drunk by waiters as they clear away the glasses.

To gauge the amount of empty space between the top of a cask and the level of liquid inside it.

Source: wiktionary.org