ullage

Ullage or headspace is the unfilled space in a container, particularly in a container for liquids.

==Etymology== The word ullage comes from the Latin word oculus, used by the Romans to refer to a barrel cork hole. This word was, in turn, taken in medieval French as oeil, from which a verb ouiller was created, meaning to fill a barrel to full capacity. Around 1300, the word ouillage was created by the Normans to refer to the amount of liquid needed to fill a barrel to capacity.