"Dark Lord of the Sith" has been used as Darth Vader's title ever since 1976, in the novelization of Star Wars. That's 6 months before the movie even came out, so the word "Sith" is older than the movies. It's as old as "The Journal of the Whills." Calling it a "prequelism" just because it was never spoken in a line of dialogue in the original trilogy ignores the fact that it's been around just as long as the original trilogy has.

What it meant was vague back then; it wasn't used to refer to the Emperor and it didn't necessarily have to be a sect of the dark side. Of course if J.J. Abrams and Disney use the word now, they are bound by the specific definition it's been given over the years.

I don't think it's enough to distinguish between words used on screen vs. words used outside the movies in tie-in materials.... I think a further distinction should be made between words that were around at the same time as the films vs. words that were invented later when the prequels came out. For example, in Return of the Jedi, we are never told in the movie what those small furry creatures on the forest moon are called, but publications outside the movie referred to them as "Ewoks," and since those publications are as old as the movie itself, nobody disputes that they are Ewoks.