Is it ensure, insure, or assure?
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 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?]