Cause you know, it's such a tricky term. E.g.: there's a question about a good intro indie game. The question's about experimental games, but some people are recommending non-experimental games from small publishers, which are really just as independent as anything anyone from the Forge or Story Games or etc. have done. But I don't like the term experimental, either. We're past experiments -- the people designing these games aren't experimenting any more. Story games? Meh. Too much emphasis on narrative games for me, although maybe that's accurate.
There may not be any good answer to this question, but I figured now is the time to ask it.