ullage

the amount by which the contents fall short of filling a container, as a cask or bottle.

the quantity of wine, liquor, or the like, remaining in a container that has lost part of its contents by evaporation, leakage, or use.

Rocketry. the volume of a loaded tank of liquid propellant in excess of the volume of the propellant; the space provided for thermal expansion of the propellant and the accumulation of gases evolved from it.