It reminds me of those very first LCD screens that were so bad you had to look at the straight on to see anything. I know what you are talking about, but I wonder myself what such an overlay is called.
Yea that's exactly what I am talking about.

. (Only portable). I just thought of a few more details. They are pretty flimsy and not hard or brittle. (They bend although if you bend them too much they will crease). They feel like plastic, but might be made out of something else. And they are used as privacy screens.
They do distort the screens color a little bit, and arn't really suited for watching movies/doing graphics work with. But are perfect for when I am on the bus doing work, or at the library working. They can be removed and placed easily too from the laptop's screen.

I think I described them as best as I can now. But maybe I'll think of even more details later.