A story told, staged or written within the work itself, which mirrors or comments on the main story.
In Hamlet
'The Mousetrap.' Marry, how? Tropically. [...] Your Majesty, and we that have free souls, it touches us not
The title “The Mousetrap” names the play performed before the court, a story inside the story that reproduces the crime recounted by the Ghost and serves to mirror the murderer.
Act III: The Mousetrap and the death of Polonius, Hamlet, to Claudius