Natural is defined by natural bodybuilding competitions, as someone who does not use prescription or illegal drugs. Thus, insulin, being over the counter, 1-test (available legally at any suppliment store), and other such things is natural, while those who use test cyp, hgh, etc are not natural.
I think of it as really a romantic notion with no absolute meaning. Personally, I do not consider the average joe to be natural. Its virtually impossible for a man to obtain a 25% bf with foods found in nature. Only by being sedintary (totally unatural for a human) and by eating processed foods, containing many ingrediates made in chemical vats, do we see this type of physique.
Also bear in mind, that even the act of weight training, with manufatured machines and weights, on a regular routine, for the purpose of building muscle mass, is not at all natural, and you are in effect, making huge changes to your hormone levels, cells, etc by using this man made devices. I do not consider it to be any less natural than injecting steroids. I only consider a natural bodybuilder (the very phrase is of course an oxymoron) to be natural if he eats absolutely no refined foods, and does not train with weights.
Of course, most people will disagree, since there is no clear or logical defination of what a natural bodybuilder or athelete is, and ANY effort to do so, will create a definition that someone can prove to be false, by finding a flaw in it.