The main namespace, article namespace or mainspace is the namespace of Wikipedia that contains the encyclopedia proper—that is, where Wikipedia articles reside.

The main namespace is the default namespace and does not use a prefix in article page names. This is distinct from other namespaces where page names are always prefixed by an indicator of the particular namespace in which the page resides. For example, all user pages are prefixed by "User:", their talk pages by "User talk:" templates by "Template:" and various types of internal administrative pages by "Wikipedia:" (such as this page). Thus, any page created without such a prefix will automatically be placed in the article namespace.

Not all pages in the article namespace are considered to be articles. The most notable exceptions are:

• the Main Page;
• for statistical purposes, some extremely short and simple pages (the criteria have varied over time);
• thousands of disambiguation pages which are used to resolve naming conflicts;
• a few million redirect pages which are used to re-route one page to another page; and
• many Wikipedia lists which serve as navigation tools rather than encyclopedic articles per se, although annotated lists—especially those promoted to "featured list" status—are regarded as encyclopedic articles.

Every page in the main namespace can have a companion talk page, and these belong to the "Talk" namespace, i.e., the pagenames start with the prefix "Talk:", followed by the name of the page in the main namespace. For example "Talk:Wikipedia" is the talk page for Wikipedia's article on Wikipedia itself.