MDL (the MIT Design Language) is a descendent of the Lisp programming language. It allows user-defined data types and had some advanced I/O features. Although MDL is obsolete, some of these features have been incorporated in later versions of Lisp.
MDL's claim to fame is that it was used to write the Zork computer game.
MDL was originally named "Muddle", and was designed for the Dynamic Modeling Group at MIT.