Source : Free On-Line Dictionary of Computing

Formal Description Technique
     
         (FDT) A {formal method} for
        developing telecomunications services and {protocols}.  FDTs
        range from abstract to implementation-oriented descriptions.
        All FDTs offer the means for producing unambiguous
        descriptions of {OSI} services and {protocols} in a more
        precise and comprehensive way than {natural language}
        descriptions.  They provide a foundation for analysis and
        verification of a description.  The target of analysis and
        verification may vary from abstract properties to concrete
        properties.  Natural language descriptions remain an essential
        adjunct to formal description, enabling an unfarmiliar reader
        to gain rapid insight into the structure and function of
        services and protocols.
     
        Examples of FDTs are {LOTOS}, {Z}, {SDL}, and {Estelle}.
     
        [ISO/IEC DTR10167: "Guidelines for the application of
        {Estelle}, {LOTOS} and {SDL}"].
     
        (1994-12-06)