A Document Type Definition (DTD) is a set of rules and guidelines that define the structure and syntax of an XML or SGML document. It determines which elements and attributes are allowed, and how they can be used within the document. DTD is essential for ensuring consistency and accuracy in document creation, and is often used in web development, content management systems, and digital publishing. Proficiency in DTD requires a solid understanding of XML and SGML syntax, as well as an ability to write and validate DTDs.