名詞: n.
the events of a character's life prior to the start of a fictional story
Flashback; Racconto; Characterization; Origin story
In narratology, a back-story (also back story or backstory) is the history behind the situation extant at the start of the main story. This literary device is often employed to lend the main story depth or verisimilitude. A back-story may include the history of characters, objects, countries, or other elements of the main story. Back-stories are usually revealed, sketchily or in full, chronologically or otherwise, as the main narrative unfold. However, a story creator may also create portions of a back-story or even an entire back-story that is solely for his or her own use in writing the main story and is never revealed in the main story. In role-playing games, a character's back-story is usually called his or her background.
The dramatic revelation of secrets from the backstory is a useful term for forming the story, recommended as far back as Aristotle's Poetics.