Dictionary.com's Word of the Year is...
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 World-Wide 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".