The Jargon File defines a "Hacker" as:
hacker n [originally, someone who makes furniture with an axe]
(Definitions 1 through 5 are correlated, and people who fit them congregate.)
- A person who enjoys exploring the details of programmable systems and how to stretch their capabilities, as opposed to most users, who prefer to learn only the minimum necessary.
- One who programs enthusiastically (even obsessively) or who enjoys programming rather than just theorising about programming.
- A person capable of appreciating hack value.
- A person who is good at programming quickly.
- An expert at a particular program, or one who frequently does work using it or on it; as in 'a Unix hacker'.
- An expert or enthusiast of any kind. One might be an astronomy hacker, for example.
- One who enjoys the intellectual challenge of creatively overcoming or circumventing limitations.
A malicious meddler who tries to discover sensitive information by poking around. Hence 'password hacker', 'network hacker'. The correct term for this sense is cracker.