Not so. In martial arts and in medicine which is far more pertinent to the discussion.
Funny thing is that tai chi also has a problem with peoples misunderstanding of a similar thing they call kua or cua ( I forget which is the right spelling).
The development of Koshi I would say is one of those fundamental 'secrets' hidden in plain sight and if you prefer a 'general' meaning for it then you will never get it.
As I said, there are instances of use for the back region, but when most people (even martial artists) use it in reference to movement they're talking about the entire pelvis and waist combined.
You can make up your own usage for it, but Japanese people won't understand it.
The kua is something quite different.