Metagame: Originally, this term was really about things that were “beyond the game”. Like, if Bob sells Park Place to Alice for $1 because he’s foolishly hoping that will improve his chances with her outside the game. In RPGs, though, we tend to use it for things that aren’t really outside the game but are merely outside the fiction of the game.
(There should be a ISO 4217 code for Monopoly money.)
In-game: This term is can be an antonym of “metagame”, and is thus subject to the same misuse by us RPGers. Of course, words mean what we use them to mean, but I think it’s worth trying to use them thoughtfully when we can. Especially if it can make our meaning clearer.
Disassociated mechanic: A common complaint about Wizards’ D&D 4e. I don’t tend to use this term, and I also tend to think that disassociated mechanics aren’t inherently bad bad.
Diegetic (and extradiegetic): Even when I use it, I can’t help but feeling I don’t really understand the term well enough to be using it.
In-character (and out-of-character): These terms seem to not cause much confusion.
So, I’m thinking I want to prefer these terms...
I kind of like “in-fiction” and “out-of-fiction”, but I think they are potentially more confusing.