Help:Transclusion

Transclusion means the inclusion of the content of one document within another document by reference. In Wikipedia transclusion, the MediaWiki software will refer to the content of one page, the template, for inclusion into the content of any other page, the target page. Similar to the way any page name can become a link by placing its name in double square brackets, any page name can become a template by placing it in double curly braces, {{Namespace:Pagename}}. Changes made to the template are then automatically reflected on all pages into which that page has been transcluded (though a purge may sometimes be necessary).

Wikipedia page on transclusion