Ullage n. : The amount which a vessel, as a cask, of liquor lacks of being full; wantage; deficiency.

Example:
Where does the Ullage (empty space) around a diamond crystal embedded in kimberlyte come from?"?

Taxonomy
ullage, liquid, barrel, tanks, liquid propellant, free encyclopedia, winemaking, container, unfilled space, expansion, gas, Steel Vessels, one side, Etymology, Wine and spirits, bung hole, search, Rocketry.

Defination from the web:
ullage came to refer to any amount by which a barrel is unfilled, perhaps because some of the contents have been used. ... Ullage is also widely used in industrial or marine settings to describe the empty space in large tanks or holds used to store or carry liquids or ...