Document Type Definition (DTD)

Document Type Definition (usage/reference to this definition is then a Document Type Declaration).

Specified in the (core) XML standard 1.0.

Describes allowed element content, attribute presence and content, their default values, defines used entities.

DTD might be either internal or external DTD (internal and external subset) or "mixed" -- both.

A document conformant with a DTD is denoted as valid ("platný" in Czech).

DTD and languages for similar purpose are denoted as modeling languages -- they model/define concrete markups.

Syntax of DTD IS NOT XML (in constrast to XML Schema and many others modeling languages).