A story told, staged, or written within the work itself, which mirrors or comments on the main story.
In the Iliad
she was weaving a great web, a double purple cloak, and in it she embroidered the many battles of the horse-taming Trojans and the bronze-breastplated Achaeans, which they suffered on her account under the hands of Ares.
The cloak Helen embroiders shows the battles between Trojans and Achaeans, that is, the very story the poem is telling. The scene within the scene makes Helen the first to represent the war as a work, and places her as a witness.