Ullage refers to the unfilled space left in containers, such as barrels, tanks, or bottles, especially when they contain liquids. This space is necessary for various reasons, including allowing for the expansion of liquid, preventing spillage or overflow during transportation, or accommodating gas that may be part of the packaging process. In the context of wine, beer, and spirits, ullage can also indicate the volume of liquid that has evaporated or leaked over time, which can be an important factor in assessing the condition and value of aged beverages. In space missions, ullage refers to the liquid propellant settling in spacecraft fuel tanks to ensure proper engine ignition in microgravity environments.