DocBook namespace is http://docbook.org/ns/docbook
DocBook V4.5:
<article> <title>Sample article</title> <para>This is a really short article.</para> </article>
DocBook V5.0:
<article xmlns="http://docbook.org/ns/docbook" …> <title>Sample article</title> <para>This is a really short article.</para> </article>
namespace serves only as an identifier—it is not dereferenced during the document processing