A hacker is someone who experiments with the limitations of systems for intellectual curiosity.
I'd say, for INTJ hackers, at least, this definition is incomplete. An INTJ must understand the system, it's our nature to do so, and the only way to understand a system is to watch it flex, to get down underneath it, around it, surround it, and know its every aspect. Because only then do you see the true boundaries -- where it cannot shift, and where it can. Only then do you achieve true understanding of how, and quite frequently, why, the system is the way it is.
The vague definition Schneier provides encompasses this quite well: "Understanding how the...system worked -- that was the true prize."