Amount by which a cask falls short of being full

• To capacity, lid off, time for the remaining liquid
• Unfilled part of a wine cask
• Amount needed to fill a cask
• Empty portion of a bottle

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
"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 ...

Wikipedia Word definitions in Wikipedia
Ullage or headspace is the unfilled space in a container, particularly with a liquid.

Wiktionary Word definitions in Wiktionary
n. 1 In a wine bottle, the empty space between the cork and the top of the wine. 2 In a cask or barrel, the empty space, occupied by air, that is created by not completely filling the cask or barrel 3 The topping-up of such a barrel with fresh wine 4 In ...

WordNet Word definitions in WordNet
n. the amount that a container (as a wine bottle or tank) lacks of being full