I can start off with my definiton of it, from what I have learned from several masters. I hope you guys can help me build and clarify where I am lacking:
1) The exact definition of enlightenment.
A) The buddha calls it "the end of suffering". I have recognized suffering in myself as the association (identification) and full immersion of belief in thought without a recognition of its surrounding space. I believe enlightenment exists in the space from which everything arises, and residing there. Concequently, spending enough time in that space gives you a great perspective on your thoughts, and also a profound sense of what jesus called "the peace that passes all understanding"
Furthermore, I agree with the buddha's definition, because it tells you what enlightenment is not. Suffering, which I have done my best to further define. The reason I agree with it, is ebcause I cannot speak for anyone else's experience, so there is a high degree of likelyhood that the actual experience of enlightenment is the same, but has similar traits or pointers/milestones to its ascertation.
2) Reasons for why enlightenment (as we define it) either is or isn't an experience.
A) Well everything that happens is an experience. I just.. Can't say more than this. It seems an arbitrary question. Does it matter at all, if we cannot experience it? I guess the question means, is it something you can think about, or is the thing that "enlightenment" refers to an experience? Thinking about it, too, is an experience, and you can definitely describe it. That much is apparent by the fact we are all here. But, they are all only a means of describing it. Like the Dao says: "The Flow of the universe is not one you can explain,
And its true name is not one you can speak." It surpasses knowledge because it is from that stillness which all knowledge arises and falls back into.
3) What else it could be besides an experience.
A) If a tree..... Heh. Like I said before, everything is an experience. Even the thoughts about it are experiences. So, it could be a concept, either perceived through thought forms, where one (like me) just describes it, or it can be feeling(s) that we get, changes in perception, yet even those can be arguably just figments of our imagination. If recent developments in science, religion, and meta-sciences (i.e. NLP) have taught us anything, is that we don't know what is real. Could it be, there is no reality, beyond what we perceive? That would be wonderful news indeed.
4) Reasons why people think it's impossible to fall from a state of enlightenment.
A) It is not impossible. You can go on and on playing kindergarten games on this question. "No, it isn't, I haven't done so." "Yes, it is, the bible says so." "No, the bible is wrong." "Well I have done so myself."
I can only subscribe to the fact that, yes, it is possible, since we were conceived from that "state" and at some point since have been removed from it. If such a large causal occurrence has happened, surely we are able to move in and out of the state of our own free will once we have achieved it again, since our will is our guide, it is our power, and before, that state was taken away without our will involved. Not willingly, of course, and I do not know, but this is my guess, by social conditioning.
Then there is the school of thought we are not born this way, and is something we later "achieve," in which case one can only speak from their experiences. I have experienced to a great degree, for short times, the states described by the masters, as I identified by certain characteristics they gave them - i.e. seeing everything as one thing, a profound sense of peace, stillness, or space, etc. Only to be taken away from it moments later. Because of this I am sure that the state is able to be taken away from you - by experience, it is the mind that does so. But, I am still identified with my thoughts. Is it possible, once the identity has been broken in such a way as, for example, Eckhart Tolle, whose effects lasted a long time? I do not know. Genpo Roshi says yes, though.
http://www.bigmind.org/Study.html In the "Path of the Human Being" he takes you through that very thing.
Enjoy@!