Quote Originally Posted by holli4pirating View Post
I would say... (the rest Dylan's long winded post here) ...
In terms of physics you are probably right. Where the term comes from in the context of breathing mechanics is that relative to air pressure, the pressure in the chest is negative during inspiration. I suppose you can't truly have a negative pressure, but as Lee pointed out, it depends on the established zero point, which in this case is atmospheric pressure.
To be technically correct, I suppose it should be called "lower than atmospheric pressure breathing" but that doesn't sound as nice.