node() matches the root node, for example in the following XPath expression:

ancestor::node()

"node() matches any node other than an attribute node and the root node"
http://www.w3.org/TR/xslt#patterns

<xsl:stylesheet version="1.0"
	xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:key name="mykey" match="/" use="'root'"/>
  <xsl:template match="key('mykey', 'root')">
	...
  </xsl:template>
</xsl:stylesheet>	

--
Oleg Tkachenko
http://www.xmllab.net
http://blog.tkachenko.com