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A subpage usually refers to a lower level web page in a website or wiki with the same main name as the starting level page plus a slash and text identifying the location of the subpage. More precisely, it refers to a part of the path in the hierarchical part of the Uniform Resource Identifier (also known as URI or URL).
For example, if the name of the main page were :first, a typical subpage would be called :first/second. In most implementations this system is recursive, so that subpages can have their own subpages (e.g. :first/second/third).
Use of subpages on Wikipedia, Wikibooks, and Wikiverisity
The subpage feature has been disabled in in main namespace (article namespace) the English Wikipedia. But subpages are used in Wikiversity, where the structure permits contributions by students at any academic level, and in Wikibooks, where chapters are subpages of a book.