An XML Schema Definition (XSD) is an instance of a W3C XML Schema. An XSD defines a type of XML document in terms of constraints upon what elements and attributes may appear, their relationship to each other, what types of data may be in them, and other things. It can be used with validation software in order to ascertain whether a particular XML document is of that type, and to produce a Post-Schema Validation Infoset.

XSDs were the first W3C-recommended XML schemas to provide a namespace- and datatype-aware alternative to using XML's built-in Document Type Definitions (DTDs).

XML Schema Definition files usually have the filename extension ".xsd". A unique Internet Media Type is not yet registered for XSDs, so "application/xml" or "text/xml" should be used, as per RFC 3023.