Mocklisp is the extension language of Gosling Emacs. While resembling Lisp in many ways, it is semantically quite different. Richard Stallman characterized it as a programming language that "looks syntactically like Lisp, but didn't have the data structures of Lisp. So programs were not data, and vital elements of Lisp were missing. Its data structures were strings, numbers and a few other specialized things." While it is true that many important language features were missing from Mocklisp, large elaborate programs were written in it, utilizing buffers and strings for complex data structures.

When Gosling Emacs became "Unipress Emacs", Mocklisp was renamed MLisp.