WordNet (r) 3.0 (2006):

intranet
    n 1: a restricted computer network; a private network created
         using World Wide Web software

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

intranet

    Any network which provides similar services
   within an organisation to those provided by the Internet
   outside it but which is not necessarily connected to the
   Internet.  The commonest example is the use by a company of
   one or more web servers on an internal TCP/IP
   network for distribution of information within the company.

   Since about 1995, intranets have become a major growth area in
   corporate computing due to the availability of cheap or free
   commercial browser and web server software which allows
   them to provide a simple, uniform hypertext interface to
   many kinds of information and application programs.

   Some companies give limited access to their intranets to other
   companies or the general public.  This is known as an
   "extranet".

   (1997-07-14)