What is the difference between narrowband and wideband?
These are just rough labels that depend on context.  Obviously, a narrowband signal has a narrower bandwidth than a wideband signal in the same context, but that's about all you can tell from that.

This is no different than "big" and "small" in everyday language.  Saying something is big or small without context is meaningless.  A small house, for example, is still a lot bigger than a big coffee cup.