or back sto·ry

[ bak-stawr-ee, ‐stohr-ee ]
/ ˈbækˌstɔr i, ‐ˌstoʊr i /

noun, plural back·sto·ries.

a narrative providing a history or background context, especially for a character or situation in a literary work, film, or dramatic series.

Origin of backstory

1980–85; back1 + story1 Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2020

Examples from the Web for backstory