/hə-ˈlō wər(-ə)ld/
interj.
1. The canonical minimal test message in the C/UNIX universe.
2. Any of the minimal programs that emit this message. 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 VAX back up yet?"