What is a namespace in XML?

Namespace in XML is a mechanism to avoid name conflicts of elements and attributes when combining XML documents from different vocabularies. It defines a resource identifier (URI) associated with a prefix used in the XML document.

Example:

<root xmlns:html="http://www.w3.org/1999/xhtml"
      xmlns:mathml="http://www.w3.org/1998/Math/MathML">
  <html:table>
    <html:tr>
      <html:td>
        <mathml:math>
          <mathml:apply>
            <mathml:plus/>
            <mathml:cn> 2 </mathml:cn>
            <mathml:cn> 2 </mathml:cn>
          </mathml:apply>
        </mathml:math>
      </html:td>
    </html:tr>
  </html:table>
</root>

Here, html and mathml are namespace prefixes associated with the respective URIs. They allow distinguishing elements table, tr, td from the HTML vocabulary from elements math, apply, plus, cn from the MathML vocabulary.

Without namespaces, if both vocabularies used, for example, the <title> element, a name conflict would occur.