XQuery and XPath Data Model
The XQuery and XPath Data Model (XDM) is the data model shared by the XPath 2.0, XSLT 2.0 and XQuery programming languages. It is a W3C recommendation and forms an integral part of all three languages. Originally, it was based on the XPath 1.0 data model which in turn is based on the XML Information Set.
The XDM consists of flat sequences of zero or more items of different types. Items can be typed or untyped and include atomic values as well as XML nodes (with elements, attributes and text nodes, etc.). Instances of the XDM can optionally be XML schema-validated.