V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

MDL
       Microstation Development Language (CAD)


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

MDL

   (Originally "Muddle").  C. Reeve, Carl Hewitt and Gerald
   Sussman, Dynamic Modeling Group, MIT ca. 1971.  Intended as a
   successor to Lisp, and a possible base for Planner-70.
   Basically LISP 1.5 with data types and arrays.  Many of its
   features were advanced at the time (I/O, interrupt handling
   and coroutining), and were incorporated into later LISP
   dialects ("optional", "rest" and "aux" markers).  In the mid
   80's there was an effort to use bytecoding to make the
   language portable.  CLU was first implemented in MDL.  Infocom
   wrote Zork in MDL, and used it as the basis for the ZIL
   interpreter.

   Implementations exist for ITS, TOPS-20, BSD 4.3, Apollo
   Domain, SunOS and A/UX.

   ["The MDL Programming Language", S.W. Galley et al, Doc
   SYS.11.01, Project MAC, MIT (Nov 1975)].