Back-story

A back-story is a literary device[?] often employed in order to lend a story depth or verisimilitude. Characters or events related to a story may only be given cursory treatment so as not to distract from the central plot development.

Probably the most extensive back-story ever created was that created by J. R. R. Tolkien for his Middle-earth stories.

See also:

• prequel
• retcon