Backstory
A backstory, background story, back-story, or background is a set of events invented for a plot, presented as preceding and leading up to that plot. It is a literary device of a narrative history all chronologically earlier than the narrative of primary interest.
It is the history of characters and other elements that underlie the situation existing at the main narrative's start. Even a purely historical work selectively reveals backstory to the audience.
Usage
As a literary device backstory is often employed to lend depth or believability to the main story. The usefulness of having a dramatic revelation was recognized by Aristotle, in Poetics.
Backstories are usually revealed, partially or in full, chronologically or otherwise, as the main narrative unfolds. However, a story creator may also create portions of a backstory or even an entire backstory that is solely for their own use.
Backstory may be revealed by various means, including flashbacks, dialogue, direct narration, summary, recollection, and exposition. The original Star Wars movie and its first two sequels are examples of a work with a preconceived backstory, which was later released as the "prequel" second set of three movies.