(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)].