In medias res
Etymology
Latin, "into the middle of things," a phrase from Horace’s Ars Poetica praising how the epic poet plunges the audience into the midst of the action rather than starting at the beginning.
noun phrase · /in MAY-dee-ahs RAYS/
Beginning a story in the middle of the action rather than at the start.
What it names
A ready handle for a storytelling choice — dropping the audience straight into events and filling in the backstory later, from epics to cold-open TV.
Example
The novel opens in medias res, mid-gunfight, and only doubles back to explain the heist three chapters on.
Related idea
Its opposite is the classical "ab ovo" — beginning "from the egg," at the very origin of events and telling everything in order.