The namespace name http://www.w3.org/2005/Atom is intended for use as per The Atom Syndication Format, a December 2005 Proposed Standard developed by the IETF atompub Working Group.
According to the The Internet Standards Process -- Revision 3,
A Proposed Standard specification is generally stable, has resolved known design choices, is believed to be well-understood, has received significant community review, and appears to enjoy enough community interest to be considered valuable. However, further experience might result in a change or even retraction of the specification before it advances.
See also URIs for W3C Namespaces. For more information about XML namespaces, please refer to Namespaces in XML.