It seems there is a difference of opinion as to what the definition of a prequel is on this question Why did Ridley Scott decide on doing a prequel? So what are the defining characteristics of a prequel? Are there varying degrees or is everything that happens previous to the first story and sharing elements ( characters, scenes, items, companies, nouns of all kinds) able to be considered a prequel?
Prequel: A story or movie containing events that precede those of an existing work.
This forms a vague descriptive definition as it does not dictate what or how far the movie precedes the story, or if the events do lead up to the film.
So to speak, Prometheus is a prequel because it takes place in the same universe as Alien and happens before its time.
Technically, Prometheus is not a prequel. Although the story is same in the same arc (universe) the plot or the film does not lead to the events of happening from Aliens, although it influences certain events. Prometheus thus is like a soft prequel, not entirely unrelated to Alien. The main thing to be a prequel is the continuum of the story. Alien is a prequel for Aliens because the two are connected by storylines and events.
I believe that a prequel, in order to be considered such, should not only take place in the same "universe" of the original movie/book/game but its plot should also be linked to the original plot, in some way. If a tale is simply set in the same universe of another one without having nothing to do with the original tale, I just cannot consider it a prequel. For instance: Star Wars I, II and III are prequels because they tell us how Anakin became Darth Vader, how the Empire was created and how Luke and Leia came to life; all these elements are the core of episodes IV, V and VI. But the events told in "Knights of the Old Republic" (written AFTER the movies and taking place 4000 years BEFORE the movies) are completely detached from the six episodes we all know. In my opinion that is not a prequel. Is just a story set in the same universe.