As nouns the difference between disappearance and fadeout
is that disappearance is the action of disappearing or vanishing while fadeout is a gradual disappearance.
disappearance
English
Noun
(s)
The action of disappearing or vanishing.
fadeout
English
Noun
(
en noun)
A gradual disappearance.
*{{quote-news, year=2009, date=February 17, author=Howard W. French, title=Surviving a Hitch in an Army of Boys, work=New York Times citation
, passage=The scene ends with a
fadeout to unconsciousness.}}