In medias res
Opening the story "in the middle of things": the action has already started; context comes later.
Starting in medias res ("into the middle of things," per Horace) means opening in full action, with no prior exposition: the chase has started, the argument is raging, the body is already on the floor. The reader reconstructs context on the move — and that reconstruction makes them active from line one.
It's contemporary fiction's dominant opening technique, a reaction against the classic novel's long expositions. Its flip side: starting too hard with no anchor produces confusion, not tension. The practical rule: the reader may not know *why* something is happening — never *what* is happening.
Example
The Iliad opens in the tenth year of the Trojan War, mid-wrath of Achilles.