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: 
In *Specter of the Past*, Luke mentions to R2 his belief that a dying Jedi can "anchor" himself to a nearby Jedi, explaining how Obi-Wan stayed with him so long. He does not use a noun for the phenomenon.
According to Wookieepedia:

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> Through special training, certain beings were able to appear as Force spirits through the Force.[1] 
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So according to official canon wiki, they are known as Force Spirits or Force Ghosts.