Manual:$wgMetaNamespace

Details
The name of the meta 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".

Take care: When you define a namespace or set a name of a namespace to something which contains space character you should use underline inspite of it. For example 'My Namespace' is an invalid name and it migt provide you many problems and you should use 'My_Namespace' instead of it. It is however $wgSitename doesn't have such this restriction so you must do this conversion while setting its name to $wgMetaNamespace.