Manual:Namespace

A namespace is a collection of pages which have content with a similar purpose, i.e. pages where the intended use is the same. Namespaces can be thought of as partitions of different types of information within the same wiki, and keep "real" content separate from user profiles, help pages, etc.

Technical overview
Pages exist within a namespace, and this can be distinguished using the namespace prefix of a page, which forms part of the title of a page, separated with a colon (:).

For example:

The "main namespace" does not have a prefix. Also, pages in the main namespace cannot have names starting with any of the existing namespaces prefixes followed by a colon.

Built-in namespaces
MediaWiki ships with 18 built-in namespaces.

The following 8 namespaces all have associated discussion namespaces.

2 other namespaces have negative indexes and have special purposes. You cannot create or delete pages in these namespaces, and there are no corresponding discussion namespaces.

Custom namespaces
It is possible to define additional namespaces to group additional classes of page together. This is done using the $wgExtraNamespaces configuration variable.

For a thorough overview, see Manual:Using custom namespaces.

Functionality
Some namespaces, like Project and Help, only exist for convenience and do not have any extra functionality compared to the Main namespace.

Template namespace pages can be transcluded without namespace prefix.

User namespace pages are associated with user accounts or IP addresses. Changes to user talk pages generate an automatic message notification.

Pages in MediaWiki namespace are used to override default messages in language file and can only be edited by administrators.

Image and Category pages have special functionality, as described in Help:Managing files and Help:Categories. Pages in these namespaces cannot be moved (renamed).

All pages in discussion namespaces have "post a comment" feature (see meta:Help:Section).