Not only to get into a system (hacker), but anything that one must do to find some piece of information that is unknown to that person (i.e. information that is not readily available or given out via the mfg. company). Also, making a change in software/hardware that increases(/possibly decreases) functionalitiy, whether by permission, is a hack.
Changing things in the Linux kernel, for example, is not wrong (illegal), however, is hacking.
