What is the difference between voltage UNBALANCE and voltage IMBALANCE?
I'm not sure there is a precise definition for you, and if there is it's likely not well known enough to be applied accurately. 

To me the difference is: 

Unbalance describes a change, meaning it was balanced and it is now not currently balanced. It's primarily a verb, by doing it you cause something to be out of balance. But it is also a noun (unbalanced) and in that form is synonymous with imbalance.

Imbalance describes the state of being not in balance, it may, or may not be desirable. This is a noun, not a verb, though some may incorrectly use it that way.