Technique
In Medias Res
From Horace, Ars Poetica (c.19 BC): advice to the epic poet. A structural convention of classical epic adopted by Milton (Paradise Lost opens after the Fall of the Angels).
Definition
Beginning a narrative at a critical point of action already in progress, without prior exposition.
Example
Homer's Iliad opens in the tenth year of the Trojan War — the back-story emerges through speech and flashback.