Definition from the Jargon file:
cracking n. [very common] The act of breaking into a computer system; what a cracker does. Contrary to widespread myth, this does not usually involve some mysterious leap of hackerly brilliance, but rather persistence and the dogged repetition of a handful of fairly well-known tricks that exploit common weaknesses in the security of target systems. Accordingly, most crackers are only mediocre hackers.
See also hacking.