What's the word I'm looking for?

I'm trying to think of what to call a part of a book that is a lot shorter than other parts, but embodies multiple chapters and is required for the plot to continue. I was thinking interlude, but I don't think that's it. (If you're not sure what I'm thinking of, think of it in terms of this: in X-Files, between seasons 5 and 6, the first movie takes place. NOT the piece of garbage that was the second movie, the first one)

Interpolation, perhaps?

Sounds about right. I'll go with that. Thanks.

if its a change of pov, interlude can work. If you build the book out of several of them, arc or simply Part can work.

Well, it's shorter than an arc or part, but longer than a chapter.

arcs don't have to be big, i've read 70 k word novels where the author has tagged three parts as arcs.

I have some chapters like this in my second book - short, weird pieces from the dead MC's POV, but kinda important, I do actually think of them as the "interlude" chapters, so your first instinct might not be wrong.