Bruce Schneier explains it in What is a hacker? :
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.
Wait, there’s more:
For years I have refused to play the semantic “hacker” vs. “cracker” game. There are good hackers and bad hackers, just as there are good electricians and bad electricians. “Hacker” is a mindset and a skill set; what you do with it is a different issue.
And I believe the best computer security experts have the hacker mindset. When I look to hire people, I look for someone who can’t walk into a store without figuring out how to shoplift. I look for someone who can’t test a computer security program without trying to get around it. I look for someone who, when told that things work in a particular way, immediately asks how things stop working if you do something else.
We need these people in security, and we need them on our side.
Is it any wonder that Bruce Schneier is like such an expert on security? Duh.
For your convenience, here’s the link again.