Ullage or headspace refers to the unfilled space in a container, particularly with a liquid.
The word comes ultimately from the Latin oculus, “eye”, which was used in a figurative sense by the Romans for the bung hole of a barrel. This was taken into French in the medieval period as oeil, from which a verb ouiller was created, to fill a barrel up to the bung hole. 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.
Wine and spirits 
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. 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.
In liquid rockets, ullage is the space within a fuel tank above the liquid propellant. This term derives from the term 'ullage' in winemaking, where it refers to the space above the liquid in a container such as a barrel or wine bottle.
Liquid, cryogenic rockets keep their propellant in tanks. These tanks are never completely filled in order to allow for the expansion of the cold liquid propellant. On the ground, the space between the top of the propellant load and the top of the tank is known as "ullage space".
In micro-gravity conditions, the liquid may float away from the engine intake, which is typically very undesirable. Small rocket engines are sometimes used to settle the propellant prior to the main engine ignition. These are called ullage motors. Used as a verb, "ullage" refers to the procedure of accelerating a space vehicle to settle liquid propellants to the bottom of their tanks before pumping the propellants into the primary engine.
Industrial use 
Ullage is also widely used in industrial or marine settings to describe the empty space in large tanks or holds used to store or carry liquids or bulk solids such as grain. In accordance with IMO regulations, the Code of Federal Regulations, and the ABS Rules for Steel Vessels, certain pressurized tanks on steel ships may not be filled greater than 98% full, although there are exceptions. This is so that the pressure relief valve is always in contact with a gas or vapor. Certain pressure relief valves for chemical use are not designed to relieve the pressure when in contact with liquids.
In some cases, the ullage in a ship's hold can be relevant to stability; liquid or dry bulk cargo in a part-filled hold can shift asymmetrically towards one side as the ship heels to one side and the other, reducing the margin of stability when compared with a full hold. Excessive ullage in a tank may contribute to the free surface effect. When referring to the free surface effect, the condition of a tank that is not full is described as a "slack tank", while a full tank is "pressed up".
- ABS Rules for Steel Vessels 2007, Part 5C
- Soroka, W. Illustrated Glossary of Packaging Terminology (Second ed.). Institute of Packaging Professionals.
- COAST GUARD BALTIMORE MD FIELD TESTING AND DEVELOPMENT CENTER: Grain ullages
- Ship inspection maritime guide.