What makes a movie 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.

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