WordNet (r) 3.0 (2006):

natural language
    n 1: a human written or spoken language used by a community;
         opposed to e.g. a computer language [syn: natural
         language, tongue] [ant: artificial language]

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

natural language

    A language spoken or written by humans, as
   opposed to a language use to program or communicate with
   computers.

   Natural language understanding is one of the hardest problems
   of artificial intelligence due to the complexity,
   irregularity and diversity of human language and the
   philosophical problems of meaning.

   See also Pleuk grammar development system, proof, MIT
   "Start" Project (http://start.csail.mit.edu/), New York U
   (http://nyu.edu/pages/linguistics/ling.html).

   (2011-01-30)