The Free On-line Dictionary of Computing (30 December 2018):

Vienna Definition Language
VDL

   (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)].