Source : Free On-Line Dictionary of Computing

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 {World-Wide Web}.  This is in
        contrast to, and usually in addition to, the company's public
        {web site} 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)