What precisly is a bone?

A lot on people talk about Poser bones. I spend a lot of time messing around inside Poser libeary files, but have never seen anything named "bone" in any one of them. Of couse I know that it has something to do with the Setup Room (which I never use), and those triangular icins.

But what in precise technical terms is a bone? Is it the entire actor in all sections of the cr2; morphs, crease angle, 'addChild' stuff, weld statement, geometry assignment,  the whole kit and cboodle? Or is it some subset of the above, and if a subset, what exactly is included in, or excluded from the subset?

I have wondered about this for a long time. I've been kind of embarrised to ask. I assumed that one day the meaning would become clear to me, but it never has. So please, your best stab at a precise technical definition of the word "bone".