A language specification by the W3C that defines an XML vocabulary for defining the contents and structure of XML documents. XML Schema is a successor to XML Document Type Declarations (DTDs), but is more expressive and better designed for expressing a type system.
XML Schema Definition (XSD), an instance of an XML schema written in the XML Schema language. An XSD defines a type of XML document in terms of constraints upon what elements and attributes may appear, their relationship to each other, and what types of data may be in them.