There is a slight difference in how the term is widely used in spaceflight, which I think may not be captured in the existing sense no. 4, which currently readsː
Noun
ullage (countable and uncountable, plural ullages)
...
4. In an industrial setting, the empty space in a tank, such as for fuel.
In spaceflight, and in particular in the microgravity conditions of a typical orbiting spacecraft, the liquid- and gas-phase components of any propellant tank (or, really, of any tank in general that has liquid and gasses in it, eg, water, sewage, some food or beverage storage, etc.) will NOT have any clearly defined ullage space, in general. The liquid phase and gas phase components will both be rather randomly spread around the total volume of the tank. There is still (in the lay language terms used in sense 4) an empty space in the tank; it is just all over the tank, and in somewhat random and unknown places. There is no defined or clear space that is "empty" in the sense the Wikitionary article is using "empty"; yet all of the many random spaces that are not liquid are "empty" (in the loose Wiktionary sense used her in this entry). But it is definitely NOT ullage in the spaceflight domain..
(ASIDE: it should probably be noted that, in the ordinary terrestrial sense when the term is used, like the wine bottle (as in sense 1) or any tank (as in sense 4), the space is not really "empty". It is just the space occupied by gas, rather than liquid. But that is a whole different topic.)
However, and this is where the term is much used in the spaceflight domain, in order to utilize a chemical-propulsion rocket engine for some particular burn (typically, this is only a very small part of the time of total on-orbit time), it is critical to add a limited but consistent force to the spacecraft for some period of time (typically seconds or minutes) in order to to settle the propellants to the engine-propellant-intake end of the tank. This action, while the small force is maintained, actively creates an ullage space at the opposite end of the tank. This small force is typically created by some (much smaller) thruster that is a part of the "reaction control system" of the spacecraft. That is how the term is used in this domain. But in general, there is no ullage space in a propellant tank until an action is taken to create it.
I have no idea how to reflect this subtle difference in the usage of ullage in spaceflight here in a Wiktionary entry, but thought it might be useful to bring it to the attention of the many good wordsmiths and dictionary writers who frequent these parts. Cheers. ̴̴̴̴N2e (talk) 22:09, 18 August 2020 (UTC)