"What is hacking?"

The definitive source for the answer to this question is the The Meaning of `Hack' section of The New Hackers' Dictionary.

:hack:  1. /n./ Originally, a quick job that produces what is
   needed, but not well.  2. /n./ An incredibly good, and perhaps very
   time-consuming, piece of work that produces exactly what is needed.
   3. /vt./ To bear emotionally or physically.  "I can't hack this
   heat!"  4. /vt./ To work on something (typically a program).  In an
   immediate sense: "What are you doing?"  "I'm hacking TECO."
   In a general (time-extended) sense: "What do you do around here?"
   "I hack TECO."  More generally, "I hack `foo'" is roughly
   equivalent to "`foo' is my major interest (or project)".  "I
   hack solid-state physics."  See {Hacking X for Y}.  5. /vt./ To
   pull a prank on.  See sense 2 and {hacker} (sense 5).  6. /vi./ To
   interact with a computer in a playful and exploratory rather than
   goal-directed way.  "Whatcha up to?"  "Oh, just hacking."
   7. /n./ Short for {hacker}.  8. See {nethack}.  9. [MIT] /v./ To
   explore the basements, roof ledges, and steam tunnels of a large,
   institutional building, to the dismay of Physical Plant workers and
   (since this is usually performed at educational institutions) the
   Campus Police.  This activity has been found to be eerily similar
   to playing adventure games such as Dungeons and Dragons and
   {Zork}.  See also {vadding}.

   Constructions on this term abound.  They include `happy hacking'
   (a farewell), `how's hacking?' (a friendly greeting among
   hackers) and `hack, hack' (a fairly content-free but friendly
   comment, often used as a temporary farewell).  For more on this
   totipotent term see "{The Meaning of `Hack'}".  See