Any attempt to compromise a network’s security. Hackers are generally curious programmers who enjoy getting into the nuts and bolts of how computer systems (usually UNIX and Microsoft Windows NT) and networks (usually TCP/IP) work. Hackers usually guard their identity carefully and use purely technological means to try to break into a system.
Hackers generally find sufficient satisfaction with the mere act of defeating a network’s security system, and they pride themselves on not damaging or taking advantage of the data stored in the systems they hack. This unwritten “hacker’s code” distinguishes hacking from cracking, which is the attempt to steal revenue from software publishers by removing software protection functions such as expiration dates and installation codes, and from phreaking, a more destructive activity that usually involves both technological hacking and some form of deception through personal contact with the target organization with the aim to defraud.
The term “hack” is also used to refer to any temporary or inelegant solution to a networking problem, as in, «I hooked in the old hub as a repeater just as a temporary solution - I know it’s a hack, but it should do fine until we can buy something better».
Microsoft Security home page