Manual:$wgNamespaceAliases

Details
These are alternate names for the primary localized ns>Special:MyLanguage/Manual:Namespace|namespace names, which are defined by   and the language file.

If a page is requested with such a prefix, the request will be redirected to the primary name.

Set this to a map from namespace names to IDs.

Example:

To add a single alias:

If you need an alias for the project you probably need a similar alias for its talk page and you should add an array as above or two lines:

Prior to MediaWiki 1.17, it was impossible to make an alias to NS_MAIN.

If this setting is set to a namespace that does not exist (e.g. the namespace has not been added to  but a constant has been defined) the alias will redirect to the main space.

Do not use spaces
Use underscores instead of spaces in namespace names.

'My Namespace' is an invalid name; use 'My_Namespace' instead.