Pseudo-namespace

A pseudo-namespace is a group of pages that users treat as a distinct namespace, but that MediaWiki recognizes it as part of another namespace (usually mainspace). This is often caused by user misunderstanding about what a namespace is.

Usually namespaces are formally recognized by MediaWiki. There are several benefits provided when a namespace is recognized by the software:
 * Special:Recentchanges can display changes to one specific namespace. When pseudo-namespaces are kept in mainspace, this means that users see changes to articles that they aren't interested in.
 * Special:Search can display search results from only specific namespaces.

To retain these features, it's better if a wiki can be configured to recognize new namespaces, rather than informally creating them.

Alternate definition
Less often, "pseudo-namespace" refers to local namespaces that don't have any content associated with them, that are generated on the fly, and internally use a negative number. The only two namespaces like this are the Special: and Media: namespaces.