InterWiki
InterWiki Links
An interwiki link links a page to a page on another website. Unlike the name suggests, the target site need not be a wiki. These are in the same form as wikilinks, but takes a prefix which specifies the target site. The prefix can be hidden using the same piped syntax as wikilinks.
Usage: [[interwikilink:page|name of link]]
Example: [[Google:AboutUs]] = Google:AboutUs
From a developer's view, interwiki links can be easily added using the instructions on MediaWiki.