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]

WordNet (r) 3.0 (2006):

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

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.