Arthur, your last post touches on what I was wanting to say while reading this thread. As for other people, "I know it when I see it." But my primary determining factor is whether I can conceivably imagine that I am all alone or that I am the first person to be there.
This typically means that areas within easy sight of cities are out, even if they are brushy and overgrown. It also means that I might consider an area with trails "wilderness" if I don't notice them and if I see no one else.
It is the thought of being able to say, with a straight face, "we're the first people to be here" and maybe make yourself actually believe it for a few seconds.