I could not think of any use for the word "ullage" until I found this on the Internet"

Basically, the ullage is the gaseous part of a container when partially
filled with a liquid. A sealed soda bottle has a small ullage of C02 at
the top. Rocket tanks often contain a pressurized ullage of helium to help
deliver the propellant to the engine under pressure. And helium is often
chosen because it's light and non-reactive.

In zero-G, there is no "top" for the ullage to rise to, so prior to an
engine restart, small jets give the rocket a small boost forward which
allows the liquid propellant to settle back to the bottom of the tank.

John Broman
Beginner satellite observer
Professional rocket builder