Manual:$wgExtraNamespaces

Details
$wgExtraNamespaces is used to configure additional, custom namespaces for a wiki.

This is covered in detail on Manual:Using custom namespaces. Note that the first custom namespace should use the ID 100, as IDs from 0 to 99 are reserved (and negative IDs have a special meaning). The maximum namespace ID was 255 on some older (pre-1.5) MediaWiki versions, 16-bit namespace numbers are supported in all current releases.

Example
To add a namespace with the name 'forum' to your wiki you need to create 2 new namespaces (one for the forum page and one for its associated talk page):