There is no definition.

MIT coined the phrase in the late 50's and it has been used by others in the computer science and engineering world every since then to describe someone who is on the bleeding edge of discovery/invention/innovaion/knowledge.

The interpretation of the word hacker that you are speaking of is the newest and is used mostly by young people, the media, and windows users, well we can add people who think they are 133t to that too.

`Hacker' by today's definition is pretty lame and does not denote wizardry the way it once did.

All the same, by the current definition, I would say it is someone who knows enough to discover/exploit flaws in an OS/firewall/software. Whether they do this for exploration, security testing, or theft seems to determine what color `hat' they wear. I would caution anyone interested in doing unsolicited security testing that lately some self proclaimed `white hat' hackers; the good guys, have been arrested by the suits.

Decide for yourself. In my opinion it is totally lame to call yourself a hacker beacuse you know a little about puters and security. This is a title that is still far better if earned by the respect of respectable peers no matter what definition you attach to it.