Source : Free On-Line Dictionary of Computing

Vienna Definition Language
     
        (VDL) IBM Vienna Labs.  A language for formal, algebraic
        definition via operational semantics.  Used to specify the
        semantics of PL/I.  See also {VDM}.
     
        ["The Vienna Definition Language", P. Wegner, ACM Comp Surveys
        4(1):5-63 (Mar 1972)].