(I’ve written some of this already in other posts; apologies for the repetition.)
I’m possibly in favour of a slightly more restrictive concept of IF than some others on the board.
The more all-encompassing the term becomes, the less useful it will be for purposes of criticism, discussion, taxonomy, and just finding games. Similar to how nearly everything on Steam seems to be tagged “adventure”, making that tag basically useless.
But I don’t really aim to attempt a watertight definition with necessary and sufficient conditions; there will be exceptions and edge cases which are difficult to classify.
With that said, I consider IF to be a form of game (or work of interactive literature) where the primary means of communication and interaction with the players is text. The players get a textual description of a situation, and they interact by typing in commands (in parser-based IF) or by choosing one of several options expressed in words (in choice-based/CYOA-style IF). The medium is essentially written language in both input and output.
When I say “textual description”, I mean that the situation is described by words as it would be in a novel, for example.
This is in contrast to being “depicted”, for lack of a better word, which is what the typical ASCII roguelikes do: they depict a situation by means of text in lieu of graphical symbols - usually even single letters, not words. So this is the reason (or one of them) why roguelikes don’t count.
There might be graphics for embellishment, but they are not essential and do not form a very important part of the experience.
This excludes graphical games, or at least the vast majority of them, especially graphical point-and-click adventures, which are in other respects quite close to IF, both in gameplay mechanics and the narrative aspects.
It’s also the main reason why I personally wouldn’t count Planescape: Torment or Disco Elysium as IF. The gameplay in them involves moving around a graphically represented world, interacting with it and being immersed in it, and it would be quite a different experience without all that.
The fact that text plays an important role in some graphical games, and that some of them could probably be adapted to a textual format with good results, does not count as a decisive criterion, in my view.
To use an analogy, it would be like the difference between a movie and a radio play. Some movies could be adapted very easily into radio plays, but that doesn’t mean they’re the same.
To be clear: not being IF doesn’t make a game less worthwhile or less interesting, just not IF. Not everything needs to be IF to be fun, engaging, and narratively engrossing.