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.
In 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.
But the whisky, of which they had drunk a considerable quantity judging by the ullage in the bottle that stood between them on the cabin table, had loosened Wyatt's tongue and exposed his innermost preoccupations.
-- Richard Woodman, The Darkening Sea
Men, like himself, sapped skeletal by ill-health, the ullage of all that was vital.
-- John Lawton, A Little White Death
The ultimate source of ullage is the French ouil, "eye or hole," which comes from combination of the Latin oculus, "eye" and the suffix -age, designating an abstract condition.