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
All pages exist within a namespace. Internally, a namespace is a numerical index which is used to store pages within the database, and is a component of the title of a page.

Broadly speaking, all pages in a namespace other than the main namespace (see below) have a prefixed title; this prefix, known as the namespace prefix, is the human-readable identifier for a namespace. Namespace prefixes can be translated, and all namespaces have a canonical prefix.

Most namespaces have an associated discussion namespace (or "talk" namespace). All discussion namespaces have odd-integer indices, and the discussion namespace index for a specific namespace with index $$n$$ is $$n + 1$$.

Built-in namespaces
MediaWiki ships with 10 built-in namespaces, of which 8 have associated discussion namespaces, for a total of 18 built-in namespaces.

Two of the built-in namespaces have negative indexes and have special purposes. All negative namespace indices are reserved for "special" purposes. You cannot create or delete pages in these 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.