Meaning True Infinite Second Jump.
ISJ would just be a move buffering/canceling into the second jump once.
TISJ would be moves that can buffer/cancel into second jumps as many times as you want.
All attacks can ISJ, but some are too slow to be TISJ'd. Do you get what I mean? x_x I don't feel like I'm explaining myself very well.
Yeah, interchanging ISJ with true ISJ to distinguish between doing it infinitely vs doing it only once is a bit too confusing. I think we could reserve the term ISJ for attacks that can be done infinitely, and simply accept the fact that you can do a single instant second jump with
any aerial in the game provided you have the precise timing. So Ganon cannot ISJ with his Fair, but he
can do a single instant second jump with the right timing. But Ganon can presumably ISJ with any of his other aerials.
Or maybe we could change the terms to something like "lagless second jump" (LSJ) to refer to the second jump itself, but "infinite lagless second jump" (ILSJ) for when attacks can be infinitely done with this. Seems less confusing that way.
EDIT: And after thinking more about it, I think the uses of this are more limited to what I initially thought. It would definitely be good for juggling with specific moves (like Marth's Fair), and it may help approaching as well as mindgames, but it may not exactly be the new wavedash or anything. And its still really f'ing hard to do :/