undefended and hanging

I would say that is about right.

Don't make it a habit to bump your own posts, btw. It isn't good etiquette.

That said, from what I've been learning, I think you have the definitions on the right track. I believe more correct ways of calling a piece "undefended" or "hanging" are these:

• Undefended: your opponent threatens a piece of yours, but your piece currently has no defenders to trade with the attacker should it be taken on the next turn.
• Hanging: you intentionally or unintentionally placed, or left, a piece in a position where it will be in an undefended state during your opponent's turn.

An undefended piece is unguarded and may or may not be under attack.

A hanging piece is unguarded and is (or will be) under attack.

So a hanging piece is a sub-class of the undefended piece.

+1 Reminds me of John Ven's descriptions of sets and sub-sets. 

Some chess players disagree on terminology, but this definition @blueemu gave is the one I deem correct as well. 

We should also include the chess term "en prise" which is incorrectly used interchangeable with "hanging", but in my opinion, "hanging" is more often from the perspective of overlooked and undefended whereas "en prise" is something left undefended intentionally (usually tactically defended or "poisoned")

     There is also an English term "hanging pawns" which means two side-by-side pawns with no pawn support on either side.