This is most commonly encountered as a technical word of the beverage technologist, the wine merchant or publican, meaning the unfilled space in a barrel or wine bottle.
The word comes ultimately from the Latin oculus, an eye, used in a figurative sense by the Romans for the bung hole of a barrel. In the medieval period this was taken into French as oeil, from which a verb ouiller was created, to fill a barrel up to the bung hole. (When wine ferments in the barrel, there’s a slow loss of liquid due to evaporation through the wood. It’s important to keep the barrels full, as otherwise undesirable bacteria and yeasts can get in and cause unwanted side fermentations.) In turn, a noun ouillage was created, which was the immediate source of our word, first recorded in Norman English about 1300, at first in the sense of the amount of liquid needed to fill a barrel up to the bung hole.
By an obvious extension, ullage came to refer to any amount by which a barrel is unfilled, perhaps because some of the contents have been used. And it is also applied to the unfilled air space at the top of a bottle of wine, which in this case is essential to allow for expansion of the contents as the temperature changes.
The word has been transferred into the vocabulary of rocket scientists to mean much the same: the unfilled space at the top of liquid fuel tanks, left to permit the fuel to expand with changes in temperature. The space between the top of the propellant load and the top of the tank is known as ullage space.