Type Description Language (henceforth TDL) is the name of a data type

Data type

In computer programming, a data type is a classification identifying one of various types of data, such as floating-point, integer, or Boolean, that determines the possible values for that type; the operations that can be done on values of that type; the meaning of the data; and the way values of...

 specification language

Specification language

A specification language is a formal language used in computer science.Unlike most programming languages, which are directly executable formal languages used to implement a system, specification languages are used during systems analysis, requirements analysis and systems design.Specification...

 defined in the book Implementing Typed Feature Structure Grammars.

It is a modeling language specifically used to describe an ontology

Ontology

Ontology is the philosophical study of the nature of being, existence or reality as such, as well as the basic categories of being and their relations...

 of HPSG types, these are typically used to model natural language

Natural language

In the philosophy of language, a natural language is any language which arises in an unpremeditated fashion as the result of the innate facility for language possessed by the human intellect. A natural language is typically used for communication, and may be spoken, signed, or written...

phenomena. The LinGO suite, amongst other DELPH-IN (Deep Linguistic Processing with HPSG) open-source project. software and implementations, utilize TDL.