Manual:$wgMetaNamespace

Details
The name of the meta namespace (also known as the project namespace), used for pages regarding the wiki itself. If set to false, the value of $wgSitename is assumed. An alias for this namespace is "Project". The associated talk namespace is specified by $wgMetaNamespaceTalk.

Example
If you wanted to use a meta namespace of "Project", you would put in LocalSettings.php: