A story told, staged, or written within the work itself, which mirrors or comments on the main story.
In The Red and the Black
An English traveler reported the intimacy in which he lived with a tiger; he had raised it and stroked it, but always kept a loaded pistol on his table.
The anecdote of the "English traveler" and the tiger is a small story within the chapter, and it mirrors the relationship between Julien and Mathilde, whom he strokes and fears at the same time.