This was a randomly interesting question because I'm sometimes mistaken for a "shy" person when I'm not. =) Generally, people who think I'm shy, just don't really know me... or they don't know what "shy" means.
Shyness, to me, is wanting more social interaction, but being unable to due to a self-perceived reason that doesn't have to be an inhibitor. Those are the two main components as I see it.
The reasons for it can be from fear, anxiety, lack of understanding, heightened sensitivity to environmental factors... people say a lot of things. They can all apply or not apply to each individual situation to varying degrees and varying combinations. But, in my mind, it doesn't matter as much as the fact that it's a perceived obstacle and can be overcome under the right circumstances.
If there are external reasons why someone can't engage in social interaction — like being in a remote physical location away from civilization or being in a coma — that doesn't necessarily indicate shyness, obviously. Sometimes, external reasons can be contributing factors to shyness. Words or actions from other people can cause the shy person to withdraw even more. The lack of experience in general — and, thus, positive experience — in social situations in childhood could contribute to shyness.
If someone doesn't actually want social interaction at all and they're completely comfortable with that, that's not shyness either. That's introversion, which is not the same thing.