Manual:$wgExtraNamespaces

Details
  configures additional, custom ns>Special:MyLanguage/Manual:Namespace|namespaces for a wiki.

It can also be used to rename the default namespaces.

This is covered in detail in .

Extensions should not use this setting, instead they should use the CanonicalNamespaces hook or load the namespace through extension.json (MW 1.26+, 234146).