What is a Hacker ?

Originally, a hacker was someone who makes furniture with an axe. In those days, nails were hard to come by (they had to be made, one by one, by a blacksmith), screws did not exist, and saws were only used to slice trees into beams and planks. A carpenter would use an axe to hack wood in to table legs or so, and to shape the parts in such a way that they could be joint together with glue. This takes quite some skill.

When I was working in Azerbaijan in 1997, there was a carpenter, Ali, who still worked that way, and the beds, tables and cabinets he made were better, stronger than those of his younger colleagues, who used more modern techniques. They also had a rough kind of beauty to them. Ali was a hacker.

It goes without saying that if a less skilled person would try to hack furniture with an axe, the result would be rather less

But in short : someone who enjoys doing something in a creative way and is extremely good at it - the 'something' ideally being scientific, technological, etc.
But there's more to it. Being (or becoming) a hacker has also something to do with the hacker mind-set, hacker attitude, belonging to the hacker culture - and being acknowledged by other hackers.