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?”