Good question, Caesar. My short, curt, answer is that the former word is an adjective and the latter word is a noun.
More seriously, in these contexts, I think that "natural" and "realistic" can be interchanged, as in my above sentence, "The non audiophile who attends acoustic concerts, or has a child taking piano lessons, knows what a piano sounds like and he will know how to answer the question of whether or not a stereo sounds natural (or realistic)". I do not think one term is better or worse than the other, but they do have slightly different meanings to me.
To me "natural" implies in this audio descriptor context, that something sounds believable, convincing and reminds one of the way the real thing or instrument sounds. It does not sound mechanical, or artificial, but rather of nature, organic, like in life. There are degrees, and some systems sound more natural than others. Some don't sound natural at all and utterly fail and creating a believable or convincing illusion or music being played in front of us. I do not mean to imply that an audio system sounds "real", that is, indistinguishable from the real thing, instrument, or reality. Our audio systems produce a man made sound, so we know it is not a real piano playing in the room in front of us, but to me, if it sounds natural, the system has the ability to suspend my disbelief just enough to sound similar to the way I remember a piano sounding while being played in a room.
I think the word "realism", that Amir used above, can indeed be the way one describes his goal in this hobby, as in "My goal is to assemble a system which can convey a sense of "realism" to the listener. I find that perfectly acceptable, but realize that some audiophiles do not have that goal and have other ways to describe what they are trying to do. I don't really know what Amir means when he uses the term.
Thanks, Peter. I guess we are back to Platonism.
Just teasing! fun thread!