infodump

or in·fo dump

[ in-foh-duhmp ]
/ ˈɪn foʊˌdʌmp /
Slang.

noun

a large quantity of backstory, or background information, supplied at once, often as a narrative at the beginning of a story, film, etc.: The first eight pages are heavy with an infodump about Harry’s childhood in Iceland.

verb (used with object)

to provide a large quantity of (backstory) at once: I like when the background is fed to me gradually, rather than just infodumped.
to provide (a reader, viewer, etc.) with a large quantity of backstory at once:If you’re going to infodump us, at least make it interesting.

verb (used without object)

to provide a large quantity of backstory at once: In the rewrite, the author infodumps, which works well for this story.

Origin of infodump

First recorded in 1975–80; info + dump