I hope this corresponds:
a history or background created for a fictional character in a film or television programme.
"a brief prologue detailing our hero's backstory"

pré-bibles : elles
proposent un résumé du concept, une présentation des personnages principaux, quelques
intentions de décors, ce que nous appelons l’arène.

--------------------------------------------------
Note added at 33 mins (2021-04-28 11:50:16 GMT)
--------------------------------------------------

Or

Elevator pitch: https://nofilmschool.com/How-To-Write-Elevator-Pitch

--------------------------------------------------
Note added at 1 day 1 hr (2021-04-29 12:26:17 GMT)
--------------------------------------------------

Bible definition here: https://www.film.vic.gov.au/glossary#page-b

A Bible tends to run around 10-20 pages, and provides an overall 'concept' pitch for a television series (usually 1-2 pages), as well as other key sections such as: episode break-down (generally between a paragraph and half a page on each episode), character profiles and major 'settings'. It provides a sense of the key plot lines that are likely to drive the series, as well as outlining the relationships between the characters over the course of the series and each of their individual journeys (or arcs).

--------------------------------------------------
Note added at 1 day 22 hrs (2021-04-30 09:21:16 GMT)
--------------------------------------------------

I have found "mini bible" used in plenty of places:

https://fundsforwriters.com/how-to-create-and-pitch-a-televi...

Create a Series Mini Bible

A mini bible gives a sweeping view of your series from top to bottom. First, it should include the title, logline and synopsis. It should also feature a breakdown of the show’s main characters, including they’re goals, flaws, quirks and motivations. After that, have a synopsis of the pilot episode followed by a list of all episodes for the first season, including a short description of what happens in each one so executives can see where your show’s headed. And feel free to spice it up with photos and illustrations throughout.

https://jillatkinson.wordpress.com/2010/03/16/how-to-success...

What’s a Mini-bible, Bible or Writer’s Guide?

Every TV series has one. But even if your own concept is still to be sold, the process of building a series bible for another series can be a giant step toward your goal of selling your own show.

I work for a native French speaker who has decades of experience working in the film industry in Los Angeles. I have translated dozens of press kits for her, and this is the term she has asked me to use here.

A Google search will show that "initial pitch bible" is commonly used.