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Definition | Hacker
TechWeb Encyclopedia

--- hacker

A person who writes programs in assembly language or in system-level languages, such as C. Although it may refer to any programmer, it implies very tedious "hacking away" at the bits and bytes.
Although it takes only a little knowledge to gain unauthorized entrance into most computers to extract information and/or perform some prank or mischief at the site, the term has unfortunately become synonymous in the popular press with "cracker," a person who performs an illegal act. This use of the term is not appreciated by the overwhelming majority of hackers who are honest professionals. See cracker, hack, samurai and script kiddie.

http://www.techweb.com/encyclopedia/defineterm?term=hacker

Free On-Line Dictionary Of Computing

--- hacker
  1. A person who enjoys exploring the details of programmable systems and how to stretch their capabilities, as opposed to most users, who prefer to learn only the minimum necessary.
  2. One who programs enthusiastically (even obsessively) or who enjoys programming rather than just theorizing about programming.
  3. A person capable of appreciating hack value.
  4. A person who is good at programming quickly.
  5. An expert at a particular program, or one who frequently does work using it or on it; as in "a Unix hacker". (Definitions 1 through 5 are correlated, and people who fit them congregate.)
  6. An expert or enthusiast of any kind. One might be an astronomy hacker, for example.
  7. One who enjoys the intellectual challenge of creatively overcoming or circumventing limitations.
http://wombat.doc.ic.ac.uk/foldoc/foldoc.cgi?query=hacker