The first element in a document. The root element is not contained by any other elements and forms the base of the tree structure created by parsing the nested elements.

The element that encloses all the other elements in an XML document and is between the optional prolog and epilog. An XML document is only permitted to have one root element.

In an XML document, the single element that contains all the content of the document.

(Application Developer's Guide - XML; search in this book)

Each XML document has one, and exactly one single root element. This element encloses all following elements and is therefore the parent element to all the other elements. The end-tag of the child element is always before the end-tag of the parent element - this is called nested elements.