The Collaborative International Dictionary of English v.0.48:

Hello \Hel*lo"\, interj. & n.
   An exclamation used as a greeting, to call attention, as an
   exclamation of surprise, or to encourage one. This variant of
   Halloo and Holloo has become the dominant form. In the
   United States, it is the most common greeting used in
   answering a telephone.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

hello
    n 1: an expression of greeting; "every morning they exchanged
         polite hellos" [syn: hello, hullo, hi, howdy, how-
         do-you-do]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "hello":
   accost, address, bob, bow, curtsy, embrace, greeting, hail,
   hand-clasp, handshake, how-do-you-do, hug, kiss, nod, salutation,
   salute, smile, smile of recognition, wave



The Free On-line Dictionary of Computing (30 December 2018):

hello, world
hello

    The canonical, minimal, first program that a
   programmer writes in a new programming language or development
   environment.  The program just prints "hello, world" to standard
   output in order to verify that the programmer can successfully
   edit, compile and run a simple program before embarking on
   anything more challenging.

   Hello, world is the first example program in the C programming
   book, K&R, and the tradition has spread from there to pretty
   much every other language and many of their textbooks.

   Environments that generate an unreasonably large executable
   for this trivial test or which require a hairy
   compiler-linker invocation to generate it are considered bad.

   Hello, World in over 400 programming languages
   (http://www.roesler-ac.de/wolfram/hello.htm).

   (2013-10-27)