Transclusion
When you put the double braces {{ }} around a page name, the program reading the wiki source code and converting it to HTML so it will display on a browser. Software will see that you wish to add the content of the named page at that point, and will go read it, convert it to html and add it at that point before continuing with converting the rest of the source code of the original page to html. That is transclusion. You refer to the page to be transcluded by naming it in braces, which is called referencing.