There are two meanings to the term.
The first meaning of hacking is the act of intruding into a system by unauthorised means. This is also in breach of the UK Computer Misuse Act
The second meaning is more benign, in that hacking means to non-maliciously alter a system so that it is more suited to the hackers' requirements. For example altering a local copy of code so that it behaves in a slightly different way to the original.
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