A wikilink is any link from one page on Wikipedia to another. The more recognizable phrase is "internal link", but internal links have a special value on a wiki-based project, and a special name.

You'll most readily recognize wikilinks as the blue-colored links on pages that take you somewhere else: usually to a different page, and sometimes to another section within the same page (like in a table of contents).

Wikilinks are also sometimes red. When you see a red link, this means there is no page to be found on the other side of the link. If you click it, you'll see Wikipedia's equivalent of a 404 message.

The basic code for making a wikilink is remarkably simple: two brackets around the phrase you want to link. To link to the page Wikipedia, write [[Wikipedia]]. In the Visual Editor, the Link button makes this even easier.