Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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