Xenologue
Also called a gaiden, this is a side story that has no consequence on the larger plot of a story. Xenologues are neither a prequel or a sequel. The content of a xenologue is generally consistent with the main story, but will not always be. Sometimes xenologues are entirely alternate stories, sometimes they are brief additions. They are often canon, but not always.
Have you seen the new xenologue? It adds a beach day in-between episode six and seven!