From Justin Hibbard’s “Lack of distributed object development delays extranets” in Computerworld (17 March 1997):
An extranet extends an intranet to trading partners, suppliers and customers via a secure Internet link.
From Robert Hertzberg’s The Raw Power of an Idea: in WebWeek (31 March 1997):
The extranet … revolves around the notion of business partners opening up their intranets to one another.
Or here’s another definition:
Internet: open access
Intranet: company access
Extranet: company, clients, partner access