What's the term or name of the literary device/technique employed when a novel is mostly written in first person from the protagonist's view, but events as yet unknown to the character are written in the omniscient third person?
It is called a frame. A frame is a literary device in which one narrative is contained within another, the outer frame being used to in some way set the scene for the other. A frame has nothing to do with person or POV directly, but it is not unlikely that the frame and the narrative it frames may be told in a different person.
Examples of frames include:
- Heart of Darkness, which includes a double frame.
- The Princess Bride, which if framed by the Grandfather telling the story to the Grandson.
- Any of the many found manuscript tales that seemed to be popular in the 19th century.
- A Thousand and One Nights, in which Scheherazade tells cliffhanger stories to avoid being killed each night.
- Canterbury tales
- Frankenstein, in which the story is told through the device of letters relating a story told to the writer of the letters.
- Amadeus, in which the story is told in the form of Salieri's confession.
It's still called "First Person". POV is determined by the majority of text. The POV is not a 'rule'. It is a type of narrative. If you were to strictly follow first-person POV it would impossible to include a scene where the first-person is not present.
In a first-person story it follows that, by default, any scene where the first person is not present is written in 3rd.
I've done this a lot because a lot of my fiction takes queues from Comic Books which do this a lot (My first inspiration were the mid-2000s Superman/Batman comics which had some great mileage out of this by showing the interal thoughts of Superman and Batman in parallel to show that both men think similarly yet distinctly for such an odd friendship. For example, in one scene, the pair have to switch cities for the night and both monolog why they hate the new city they are working in compared to their own stomping ground.).
As a rule, I typically designate one character to have the first person perspective, it its always italicized and in separate paragraphs from the 3rd person elements. Content wise, it is limited to her thoughts and reflections of the events going on around her and is not communicated to other characters... usually. It's also always told from the present character's POV, even if the memory is being portrayed in the third person. Meanwhile, anything observable or being shown to the reader is done in the third person and it is of limited omniscient. The reader is not given any thought processes of any other character unless it is verbalized. The first person may appear in scenes that she is not present in, but it shall not display any thought that can only happen from observing the scene, which allows for ironic thoughts if the thoughts are countered by the action OR giving dramatic tension by letting the reader know something is going to happen that she does not know, thus creating suspense (and impossible in first person only.).
Writing this, I tend to write the third person first, then go back and add the first person (which incidentally, is a great way to force a proof read on your work) on a chapter basis.
End of the day, I don't know if there is a name for this POV and it is limited in non-visual medium by not having any signal of change in POV in the first person elements, so it must rely on one and only one character over the course of the book.