a means of associating one or more elements in an XML document with a particular URI. This effectively means that the element is identified by both its name and its namespace URI. See http://www.w3.org/TR/REC-xml-names
A way of defining each element type and attribute name in an XML document unambiguously (through associations with specific URIs) so that two or more XML-based languages may be used in that document without creating a conflict.
a collection of element and attribute names, identified by a Unique Resource Identifier (URI), which often takes the form of a URL, but is really just a unique string, not a pointer to a web page