Is this a prequel or a sequel?

It's an amazing one, that's for sure (no smoking was involved to make such a statement).

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Rogue One: A Star Wars Pre-Sequel!

Both, but it’s a lead up to episode 4

It's neither. It's a 100% "what if" standalone Star Wars story.

It's not a prequel because the rebel plans are not transmitted / beamed aboard the Tantive IV as per ANH.

And it's not a sequel as the Death Star was seen to be designed by the insect guys in the prequels - with the construction already underway 20 years before this film!

So it's nothing.

technically, any movie or series set before A New Hope is a prequel.