The term to describe a set of related element names or attributes within an XML document. The namespace syntax and its usage is defined by a W3C Recommendation. For example, the <xsl:apply-templates/ > element is identified as part of the XSL namespace. Namespaces are declared in the XML document or DTD before they are used be using the following attribute syntax: xmlns:xsl="http://www.w3.org/TR/WD-xsl".