Question: I realize that, out-of-universe, we have come to refer to them as Force Ghosts. However I doubt Qui-Gon or Yoda would use the words "Force Ghost" to describe what happens. Is there an **in-universe** word or phraseology that Jedi would use to refer to what we call "Force Ghosts"?

Answer: 
So far in canon, no, there doesn't appear to be a specific term. They're only referenced as one would an individual. Mostly, they're not referenced at all in dialogue.

This would actually make sense since unlike in Legends, this is not a common enough phenomenon in canon for it to be discussed frequently enough to warrant a specific term. Until Qui-Gon contected Yoda in '[Voices](https://starwars.fandom.com/wiki/Voices)' it was thought impossible for someone to retain their individuality and return from the "netherworld of the force."

Even the visions of Qui-gon, Shmi and grown-up Ahsoka on [Mortis](https://starwars.fandom.com/wiki/Overlords) weren't actual "ghosts" but illusions created by the Son. Just as the [image of Darth Bane Yoda faced on Moraband](https://starwars.fandom.com/wiki/Sacrifice) was an illusion created by the Force Priestesses, who where themselves apparently actual force ghosts, though far more solid looking than we saw in tESB and RotJ.

**Addendum**: Since I gave the initial answer some more information has come to light. While it does not change anything substantive, it may add more context and indeed demonstrate that the term "ghost" may not be appropriate even out of universe. Perhaps most significantly is the short story '[Master and Apprentice](https://starwars.fandom.com/wiki/Master_and_Apprentice_(short_story))' by Claudia Gray (not to be confused with the [novel of the same name](https://starwars.fandom.com/wiki/Master_&_Apprentice), also by Gray.) In which the story is told entirely from Qui-Gon's perspective as he manifests a physical form to communicate with Obi-Wan during the events of ANH (specifically, at the Sandcrawler after Luke rushed off home.)
The key phrase there is "physical" form. Flesh, blood and bones and all; so it seems the term "ghost" isn't even apt since they're not just projections or apparitions but actual tangible (if temporary) bodies.

This actually lines up quite well with depictions in RotJ, TLJ and other more recent stories which I shall not spoil. In which transcendent force users are seen to interact with people and objects; in the former case Kenobi is seen moving vines aside before sitting down on a log, while in the latter Yoda bops Luke on the nose with his walking stick.
According to Wookieepedia:

> 
> Through special training, certain beings were able to appear as Force spirits through the Force.[1] 
> 
> 
> 

So according to official canon wiki, they are known as Force Spirits or Force Ghosts.