ullage

noun

1. the amount by which the contents fall short of filling a container, as a cask or bottle.
2. the quantity of wine, liquor, or the like, remaining in a container that has lost part of its contents by evaporation, leakage, or use.
3. 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.

noun

1. the volume by which a liquid container falls short of being full
2. 1. the quantity of liquid lost from a container due to leakage or evaporation
   2. (in customs terminology) the amount of liquid remaining in a container after such loss

verb (tr)

3. to create ullage in
4. to determine the amount of ullage in
5. to fill up ullage in

n.“amount by which a cask or bottle falls short of being full,” late 15c., from Anglo-French ulliage (early 14c.), Anglo-Latin oliagium (late 13c.), Old French ouillage, from ouiller “to fill up (a barrel) to the bung,” literally “to fill to the eye,” from ueil “eye,” from Latin oculus (see eye (n.)).