A language for addressing parts of an XML document, selecting nodes (elements, attributes, character data, etc.) by their name, their value, and/or their location relative to specified other nodes in a tree view of the document's structure. XPath is used within other XML standards, for example XQuery and XSL; indeed, XPath version 2, released in 2007, is a subset of XQuery.
http://www.w3.org/TR/xpath20 The XPath (version 2.0) specification
http://www.w3.org/TR/xpath The XPath (version 1.0) specification (still very widely used)