Ullage

This word was, in turn, taken in medieval French as oeil, from which a verb ouiller was created, meaning to fill a barrel to full capacity.

It is also applied to the unfilled air space at the top of a bottle of wine, which in this case is essential to allow for expansion of the contents as the temperature changes.

When calculating tax returns and the like, licensed premises owners, landlords or managers can factor in the duty on ullage, or unavoidable barrel wastage.

Ullage pressure is a critical measurable during powered rocket flight, because it affects tank structural integrity and engine net positive suction head (NPSH).

[citation needed] Ullage is also widely used in industrial or marine settings to describe the empty space in large tanks or holds used to store or carry liquids or bulk solids such as grain.