Wikipedia
Frame story
A frame story (also known as a frame tale or frame narrative) is a literary technique that sometimes serves as a companion piece to a story within a story, whereby an introductory or main narrative is presented, at least in part, for the purpose of setting the stage either for a more emphasized second narrative or for a set of shorter stories. The frame story leads readers from a first story into another, smaller one (or several ones) within it.
Usage examples of "frame story".
Wells's The Time Machine has a frame story, as do countless tales-told-in-a-bar.
ANOTHER [[frame story]] Like Chapter 4, this chapter is a frame story.