In medias res

In medias res means opening a story in the middle of the action already underway, rather than at the chronological beginning, and supplying the earlier events later.

The Latin means into the middle of things, and it comes from Horace's Ars Poetica, describing Homer's method. The Iliad opens in the tenth year of the war. The Odyssey opens with Odysseus already stranded and most of his voyage behind him.

Modern usage is often looser: people say in medias res when they mean the first chapter starts at a point of pressure. That is simply a strong opening, and it is not the same thing. The classical technique involves real chronological displacement — the beginning of the story is not the beginning of the telling, and the gap gets filled in as the book goes.

The common failure is a prologue-sized action sequence starring people the reader has no reason to care about yet. Starting in the middle only works when the situation is legible. The reader does not need to know how these people got here, but they do need to know what someone wants and what happens if they fail. Get that in early, and the backstory can wait — see motivated exposition.