The amount that a container (as a wine bottle or tank) lacks of being full

• Amount by which a cask falls short of being full
• Liquid lost from a container
• Amount lacking to make a full cask
• Empty portion of a bottle
• Unfilled part of a wine cask
• Amount needed to make a full cask

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 ...

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Ullage \Ul"lage\ (?; 48), n. [OF. eullage, ovillage, the filling up of a cask, fr. ouillier, oillier, euillier, to fill a wine cask; properly, to add oil to prevent evaporation, as to a flask that is nearly full, fr. OF. oile oil. See Oil .] (Com.) The ...