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. The associated talk namespace is specified by $wgMetaNamespaceTalk. An alias for this namespace is "Project".

Note: Be sure to use underscores instead of spaces, otherwise you will have trouble. (OK to use spaces in $wgSitename.)