Dear Jamshid, your questions are always stimulating (and difficoult too).
I think "illumination" can be really understood only by those who really reach it. It is said that is something that you can't reach little by little, but you just "jump in" all of a sudden.
This is how I figure it out:
We percieve the world around us with our sense, and our structure of mind. The reality is something we built from the things we think are coming from "outside", but that in truth are only the projection of our inner thought and way of "working" of our brain. We give a sense, a meaning to all this, and we can be chained by this.
I think illumination comes when we learn how to stop the mind working this way, we connect with our inner and real being, and we get in contact with the energy around us letting it goes through us. When we get free from the concept of time and space, of cause and effect, and we enter a dimension completely different. I would compare it to a looking at the moon over us through a image reflected into a mirror, and to get free from the mirror and learn to look at it without it. but the mirror is also a part of ourself, so to get rid from a part of ourself is something necessary.
I also see it as the ability to take a distance from the concept of individuality, and from concepts in general, being this just a reflection of our mind's activity.
So I should say it is a condition in which the illuminated can see things as they are free of illusions.
Of course I'm not an illuminated! so I hope not to have said something too silly.