The Jargon File (version 4.4.7, 29 Dec 2003):

hello world
 interj.

    1. The canonical minimal test message in the C/Unix universe.

    2. Any of the minimal programs that emit this message (a representative
    sample in various languages can be found at http://www.latech.edu/~acm/
    helloworld/). Traditionally, the first program a C coder is supposed to
    write in a new environment is one that just prints ?hello, world? to
    standard output (and indeed it is the first example program in K&R).
    Environments that generate an unreasonably large executable for this
    trivial test or which require a hairy compiler-linker invocation to
    generate it are considered to lose (see X).

    3. Greeting uttered by a hacker making an entrance or requesting
    information from anyone present. ?Hello, world! Is the LAN back up yet??