terminology - What is the term for an accessible character that knows nothing?

In a lot of books, movies, or TV shows, there will be a character that knows little to nothing about the subject at hand. Most cop shows and medical shows will have someone who doesn't know very simple things so that the story can be accessible by all. Another prime example of this would be Marty McFly in Back to the Future. He knew nothing about time travel (just like the audience) so Doc Brown explaining to him about the inter workings of "flux capacitors" and how fast one must go in a DeLorean to travel in time were to keep the audience from saying "heavy."

In all seriousness though, is there a term for that "literary device?" Is it a literary device?

To follow up, a few movies recently are deciding not to include that character in the interest of realism. They feel in order to keep things as true to life as possible, you would not have a doctor who didn't know simple medical terminology or a cop who didn't know the proper collection procedure. They focus on the story or relationships rather than the jargon. Is this a better way to write? Is it better to alienate some and connect with a few or write to the masses (with the character question in mind)?

tldr:

1. Is there a term for that character in many shows that knows nothing whose sole purpose for being there is so the audience can learn as he does?


2. Should you use this type of character in writing?

Answer

There is indeed such a term.

Phil Farrand of The Nitpicker's Guide to Star Trek called this "being the cabbagehead." Certain information had to be revealed to the audience, but it was information which the characters would reasonably already know.

So the writers picked someone in the room to be the "cabbagehead," meaning someone developed the I.Q. of a cabbage and everything had to be explained to him or her as though s/he had never gone through Starfleet training and years of spaceflight experience. (Counselor Troi got this role a lot on TNG.)

The cabbagehead doesn't have to be someone who abruptly turns stupid, however. (TV Tropes calls this holding the Idiot Ball.) This role can be more realistically played by a person who is on the job for the first day, someone "new in town," someone from another planet, a child, someone who didn't have sufficient clearance, someone out of the information loop, or a new in-law. These are not doctors who have forgotten med school, but people who could not be expected to know certain information, so the other characters must explain it to them (and thereby the audience).