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

extranet

    The extension of a company's intranet out
   onto the Internet, e.g. to allow selected customers,
   suppliers and mobile workers to access the company's private
   data and applications via the web.  This is in
   contrast to, and usually in addition to, the company's public
   website which is accessible to everyone.  The difference
   can be somewhat blurred but generally an extranet implies
   real-time access through a firewall of some kind.

   Such facilities require very careful attention to security but
   are becoming an increasingly important means of delivering
   services and communicating efficiently.

   [Did Marc Andreessen invent the term in September 1996?]

   (1997-12-17)