Definition of "backstory"

These backstories cast the characters' behavior in a new light: of course Gertrude would promptly marry the man who saved her from abuse; of course Ophelia would be desperate and indiscreet in seeking Hamlet's love.

2004 January 16, Kelly Kleiman, “Her Side of the Story”, in Chicago Reader‎

Inexplicably, and at great risk to the integrity of the movie, the filmmakers have burdened him with a psychological back story pulled out of a folder in some studio filing cabinet. Why does Wonka spend his days confecting sweets?

2005 July 15, A. O. Scott, “Looking for the Candy, Finding a Back Story”, in The New York Times‎

A core element of understanding inner character is backstory. In other words, where's your character come from? What's happened to them in the past that's affecting who—and how—they are now?

2012, Craig Batty, Screenplays & how to write & sell them, Oldcastle Books

Prince Harry may yet turn out to be the crown’s best insurance policy. By marrying Meghan Markle, he is linking royalty with a 21st-century celebrity who has a backstory that many people in Britain can identify with.

2017 December 27, “The Guardian view on Prince Harry: the monarchy’s best insurance policy”, in the Guardian‎