The best definition of a hacker that I have ever seen is due to Bruce Schneier, in his book, Secrets and Lies: Digital Security in a Networked World. He states in Chapter 4:
I define a hacker as an individual who experiments with the limitations of systems for intellectual curiosity or sheer pleasure; the word describes a person with a particular set of skills and not a particular set of morals.
Admittedly this sounds a lot like the way Beauvoir describes Bobby Newmark in William Gibson's Count Zero: someone who is interested in systems, how they work, and what their limits are. The term given in this definition describes some script kiddies and crackers as much as it describes the people described in the entry for "Hacker" in the New Hacker's Dictionary. And more than that as well. Ultimately, this drive to exploration, discovery, and the quest for knowledge is what makes a hacker a hacker. From the previous definitions given, dwyn's definition comes closest.
The phone phreaks of yore were true hackers in this sense; they didn't do the things they did just to annoy the telco or make free 8-hour phone calls to Manila or some random place somewhere in the world. For most of them, what they wanted was secret knowledge, understanding the esoteric knowledge of the phone system better than its designers did.
Virus writers, the designers of DDoS techniques, and crackers can also sometimes be described as hackers; for some of them probably the testing of the limits of the systems they are exploiting and knowledge of their security weaknesses are the things they are after. The fact that they usually destroy or ruin the systems they interact with they consider as the price of their knowledge. The pursuit of knowledge does not always lead to beneficial ends. Which is why this group are sometimes referred to as Dark Side hackers.
The same definition also covers the programmers who program for the sheer joy of programming. Once again, their prize is knowledge; the knowledge of how computer systems work, how computers can be used, controlled, and manipulated. It is through people like these that we have better systems today than yesterday, as they constantly test the limits of computer systems.
Many of the great scientists of the past and present can also be described with this term. They strive to understand the workings of that greatest system of all: the universe itself, what laws it obeys, how it is built, and what its limits are. They strive to know the mind of God, the greatest hacker of them all.