Mise en abyme
A work embedded within the work: a story within the story, a play within the play, mirroring the whole.
Mise en abyme embeds within the work a replica of itself: the play staged at the heart of Hamlet re-enacts the king's crime; the painter paints the painting we're looking at. The term comes from heraldry, where a coat of arms may contain a miniature copy of itself at its center.
At its best the device is no ornament but an instrument: the embedded story reveals, comments on or forks the frame story — the play in Hamlet is a trap set for the guilty. Mise en abyme also raises the vertiginous question of reality levels, favorite ground of Borges and the postmodern novel: who is reading whom?
Example
The play within Hamlet, staged to "catch the conscience of the king."