What's the literary tem for a "spin-off" that is made before the main work?

I have one novel in the works where the characters and plot points have been outlined. However, I am about to finish one another short story which also uses the same characters and setting as the novel.

If the novel is finished first and I write the short story afterwards, the short story would be called a "spin-off" of that novel.

However, in this case as the short story will be finished first, I'm not sure what to call it as technically the main work does not exist yet.

The short story would serve as a sort of "introduction" piece for the novel that is to follow. However, the storyline in the short story actually takes place after the events in the novel, so it's a bit complicated as it's not really a "pilot" or a "prequel". Anyone know of similar case with other works and can help me find the right term to call it? Thanks very much in advance.