I observed that few films have a short story in start of the film during opening credits, specially in Zack Snyder's films. In Watchmen it's about Minutemen, and in Batman v Superman: Dawn of Justice it's about Batman's parents death. Both the time it was more visual with near to no dialogue. What is the term for this kind prequelish segment during start of film?
In screenwriting for film, it's known as a 'prologue', and in your case specifically it can be referred to as a 'flashback prologue'(see below).
A prologue is defined as a separate introductory section of a literary or musical work.
In your two example movies, these prologues were structured as a 'montage'.
A montage is a collection of very short scenes, sometimes only a single shot each, designed to show a series of actions over time.
Here is an excerpt from the script(source) of The Lord of the Rings: The Fellowship of the Ring, where we can see the screenwriter(s) describing the narrated "flashback" introduction as a prologue:
(English:) The world is changed: I feel it in the water, I feel it in the earth, I smell it in the air...Much that once was is lost, for none now live who remember it. SUPER: THE LORD OF THE RINGS EXT. PROLOGUE -- DAY IMAGE: FLICKERING FIRELIGHT. The NOLDORIN FORGE in EREGION. MOLTEN GOLD POURS from the lip of an IRON LADLE. GALADRIEL (V.O.) It began with the forging of the Great Rings.
There are a few characteristics of a prologue.
#1 – The prologue scene stands alone. It’s a self-contained scene that has a beginning, a middle and an end. It’s a story within the story.
#2 – The prologue scene reflects what’s to come. It hits you in the gut by amplifying the chore emotion of the film, giving you the essence of what’s to come.
#3 – The prologue scene lays down what the film is about. It shows you the main characters, the dynamics and gives you sneak peek at the film’s theme. (man vs. man, man vs. God, man vs. self, man vs. Society, man vs. Supernatural, man vs. Technology)
Michael Hauge describes a few different forms a prologue can take
- A FLASHBACK is a sequence that takes place significantly prior to the action of the rest of the screenplay – sometimes centuries before – and may or may not involve the hero. Its primary functions are to create anticipation of what’s to come (in The Mummy we see the execution of Imhotep, creating anticipation of his returning from the dead); curiosity about how these events will involve the hero (in The Exorcist we have no idea what unearthing an artifact in the desert has to do with a mother and daughter in Georgetown); exposition that will be needed to understand later events in the story (seeing the “human element” stop the launch of the missile in War Games explains why the computer system was created); and/or a wound that will lead to the hero’s inner conflict later in the story (a young Jo watching her father swept away by a tornado in Twister).
- A BOOKEND is a sequence that takes place some time after the action of the rest of the screenplay, prior to going back in time and telling the story. A bookend carries a similar function to hearing the words, “Once upon a time…” and often includes a character who will narrate the rest of the film. This device is especially useful in biographies and period pieces, because it draws us into the screenplay before taking us back in time to a setting that might be hard for an audience to relate to. The Princess Bride, Out of Africa, The Road to Perdition and The Curious Case of Benjamin Button all employ bookend/narrator openings. Occasionally the “present-day” bookend opening continues its own story line, and the screenplay intercuts between the two, as in Titanic, Fried Green Tomatoes and The Usual Suspects (although The Usual Suspects adds the more complex combination of two flashbacks – we open at the end of the story that Verbal Kent will later tell from the beginning in the BOOKEND sequence, that takes place in the police station).
- A MID-STORY PEAK MOMENT is a sequence of high conflict from somewhere in the middle of the story. We then go back to the “beginning” of the story, which will unfold chronologically until we’re again watching the opening sequence, this time played out to completion. This peak moment elicits strong emotion because of the conflict, creates anticipation of what we know is to come, and creates curiosity about how we’re going to get there, and how the hero will ever be able to overcome that conflict. Iron Man opens this way, with Tony Stark left for dead after an attack in Afghanistan. Seven Pounds opens with Ben Thomas calling 911 to report his own suicide. And in Mission Impossible 3, Ethan Hunt is threatened with the death of his fiancée if he doesn’t reveal the information that his nemesis demands.
- A NEMESIS INTRODUCTION opens with the character who will become the greatest obstacle to the hero achieving his goal. This could be a villain (The Joker in The Dark Knight), an opponent (Apollo Creed in Rocky), a romantic rival (Cal in Titanic or Zachary in Wedding Crashers), or simply a character in opposition to the hero (Mozart in Amadeus or Raymond in Rain Man). By introducing the Nemesis before your hero, you open with immediate conflict, you create anticipation of the jeopardy that awaits the hero when she must face the far more powerful (or far more evil) Nemesis of the screenplay, and you create curiosity about what will bring them together, and how the hero will ultimately prevail. This is why we are introduced to the Joker in the opening sequence of The Dark Knight, and why we meet the cyborg from the future in The Terminator before we see Sarah Connor living her everyday life as a waitress.