I tend to agree with Matt that this is a case of family resemblance, and therefore wouldn’t attempt a watertight definition with necessary and sufficient conditions.
But generally speaking, 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/readers 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 (in choice-based/CYOA-style IF).
There might be graphics to enhance the experience, but they are not essential.
(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.)
@6f : If you browse the IF Wiki, the IFDB, and the IF Competition site, you can get a fairly good idea what interactive fiction (in this community’s sense of the term) typically looks like.
This is not meant to imply that everything else is thereby automatically non-IF, just that those comprise the paradigm cases.
And, as Matt also said, I think you don’t need to worry much regarding community interaction.
When you’re in the position of looking at other people’s projects, just keep an open mind and don’t dismiss anything out of hand as “non-IF, therefore not worth a second look”.
When you’re in the position of talking about your own projects and you’re in serious doubt, maybe try to concentrate on those aspects that your project has in common with various forms of IF. There can be worthwhile discussions about puzzles, game design, writing techniques, narrative structure, pacing, player engagement, and whatever else, even if your project is not a typical case of IF. Maybe it’s an interesting hybrid that brings together the best of several genres/worlds, who knows.
(And if you feel it would, after all, be off-topic, then there’s still the off-topic section of the forum.)
Have fun, and welcome to the community!