ullage

from WordNet (r) 3.0 (2006)

ullage
    n 1: the amount that a container (as a wine bottle or tank)
         lacks of being full

from The Collaborative International Dictionary of English v.0.48

ullage \ul"lage\ ([u^]l"l[asl]j; 48), n. [OF. eullage, ouillage,
   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 amount which a vessel, as a cask, of liquor lacks of
   being full; wantage; deficiency.
   [1913 Webster]

from Bouvier's Law Dictionary, Revised 6th Ed (1856)

ULLAGE, com. law. When a cask is gauged, what it wants of being full is 
called ullage.