The Free On-line Dictionary of Computing (30 December 2018):

XML schema

    XML data that describes the relationship between
   elements and attributes in some other class of XML data.  A
   schema may or may not include data type representations.  XML
   schemas are a more advanced alternative to DTDs.

   (http://vbxml.com/conference/wrox/2000_vegas/text/kurt_schema.asp)

   (2006-08-23)