The word hacker is usually related to computers and computer networks, and it even appears in the Internet Users' Glossary (RFC 1983) with the following definition:
hacker: A person who delights in having an intimate understanding of the internal workings of a system, computers and computer networks in particular. The term is often misused in a pejorative context, where "cracker" would be the correct term.
Computers and computer networks are explicitly mentioned, but "in particular" and not as the only possibility. Bruce Schneier goes beyond this, giving a more general definition, with no reference to that specific field:
A hacker is someone who thinks outside the box. It's someone who discards conventional wisdom, and does something else instead. It's someone who looks at the edge and wonders what's beyond. It's someone who sees a set of rules and wonders what happens if you don't follow them. A hacker is someone who experiments with the limitations of systems for intellectual curiosity.
This is maybe a bit provocative (as Schneier says in his article, which I recommend, Galileo was a hacker!), but I think it gets the point: it is not just a matter of technical skills, it is most of all a mindset. I recommend another reading: The Hacker Ethic and the Spirit of the Information Age by Pekka Himanen.
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