A more thorough explanation:

Definition: Ullage is the amount of space left in a container, like a cask or a barrel, when it is not completely full of liquid.

Example: If a cask can hold 100 liters of wine, but it only has 90 liters of wine in it, then the ullage of the cask is 10 liters.

Explanation: Ullage is the term used to describe the empty space in a container that is not filled with liquid. In the example given, the cask has an ullage of 10 liters because it is not completely full of wine. This term is often used in the wine and spirits industry to describe the amount of space left in a container after the liquid has been filled to a certain level.