Computer hackers are a subset of that. They access or attack systems via routes that were not designed for them to use and exploit in that way. Coding hackers use existing code and modify it for another purpose. I don't see a coder who starts with a blank page and libraries or APIs as a hacker as such. They are creators, using tool provided for intended purpose.
Too widen it: an art hacker, for example, would take old bits of metal and make a sculpture. An artist starts with a blank page and raw materials. People who fix cars with anything around them in Africa or Cuba too are hackers. Im not sure of the details, but when Apollo whatever number went wrong and they had to help the astronauts get home, that was hacking. People who fit V8 engines to small FWD cars; hackers.
I also think its about targeted expertise, rather than over all expertise. Hackers learn the small part they need to re-task, but don't have over all knowledge. As well as that, I think its about cutting through the surrounding crap. Getting the job done regardless of convention, rules, regulation, and procedure. So, hacking is about how you get the job done.
That is how I see it, but of course in all that is room for debate and many fine lines to argue over.
Ultimately to me, its a universal mentality. Macgyver was an ultimate hacker!!!!