or back sto·ry
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
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
the events which take place before, and which help to bring about, the events portrayed in a film
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
c.1990, from background story.
Online Etymology Dictionary, © 2010 Douglas Harper