A DTD (Document Type Definition) file is a specialized text file used to define the structure, legal elements, and attributes of an XML, SGML, or HTML document. It acts as a blueprint or a set of rules that a document must follow to be considered 'valid' according to that specific definition. By defining the nesting of elements, the frequency of their occurrence, and the types of data they can contain, a DTD ensures consistency and reliability when data is exchanged between different software applications or organizations. Although DTDs were the original method for defining XML schemas, they have some limitations, such as a lack of support for complex data types and XML namespaces. Consequently, many modern developers prefer XML Schema (XSD) or Relax NG for more robust validation. However, DTDs are still widely used in legacy systems, publishing workflows, and for defining the structure of standard formats like EPUB or older versions of HTML. Because they are plain text, they can be edited in any basic text editor or specialized XML development environment.