What... is an Unbirth?

What can be gleaned of its intent through the name alone? It's confusing. I really think they could've chosen a better name.

Anyway, in regard to what the creatures might look like. Based on what we've seen of Heartless and Nobodies, there would likely be primary supreme Unbirths and of course grunt-like Unbirths. My theory is that either one will definitely look like children. That was my theory when I came up with the Grim: the group I was going to introduce in my KH fanfic to complete the trifecta. Like the Nobodies, the Unbirths will have identifiable superiors but unlike their adult counterparts, they won't be uniform. Any Unbirth grunts that may exist would be considered too young to be able to express their own individuality and thus come across as smaller versions of common Nobodies or perhaps even Shadow Heartless. (anybody ever wonder if Vivi was a Shadow?)

I actually created the Grim with the hope of having Axel return in some form. He was cool and I was disappointed when he faded away. I hope the Unbirths live up to my expectations and I really hope that at least one member of the Organization will return in this form. (likely that would be Zexion)