Empty portion of a bottle

• Amount of liquid lost in shipping
• Amount needed to fill a cask
• Amount by which a cask falls short of being full
• Unfilled part of a wine container
• Amount needed to make a full cask
• Unfilled part of a wine cask
• Result of liquid loss in a cask

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

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 ...