A hack is a clever, unanticipated use or repurposing of something. It's also solving a problem via a method which is expedient and clever, but inelegant or otherwise flawed: a hack as a clever kludge.

It's that second definition that makes me reluctant to self-identify as a hacker. To my mind, hackers produce hacked-together software, which is unpolished and jury-rigged. As programmers, we should be sheepish about our hacks, and seek to correct them - not wear them proudly.