ullage
noun
ul·lage ˈə-lij 
: the amount that a container (such as a tank or cask) lacks of being full
Example Sentences
Recent Examples on the Web McDowell says there are at least 64 ullage motors currently in orbit. 
Joshua Hawkins, BGR, 6 May 2022 Russia used these ullage motors to help accelerate the parent rocket stages slightly, according to Jonathan McDowell. Joshua Hawkins, BGR, 6 May 2022
Joshua Hawkins, BGR, 6 May 2022 Russia used these ullage motors to help accelerate the parent rocket stages slightly, according to Jonathan McDowell. Joshua Hawkins, BGR, 6 May 2022 Word History
Etymology
Middle English ulage, from Anglo-French ulliage, from *ullier to fill a partially empty cask, from Old French (Picard dialect) oel bunghȯle, literally, eye, from Latin oculus eye — more at eye
First Known Use
15th century, in the meaning defined above