define.xml and SDTM

Define.xml is the document which specifies the standard for providing Case Report Tabulations Data Definitions in an XML format for submission to regulatory authorities (e.g., FDA). 

The XML schema used to define the expected structure for these XML files is based on an extension to the CDISC Operational Data Model (ODM).

In the picture below the key aspects are highlighted with the red text, basically it is a listing of data items that a sponsor will transmit to FDA along with the regulatory application.

Each data set if furthered detailed with what data items are sent and their nature (ex: Derived or from a CRF page)

You can find the DTD definition and stylesheets to transform define.xml on CDISC website : http://www.cdisc.org/models/def/v1.0/index.html