Emacs Lisp, usually shortened to elisp, is the programming language in which GNU Emacs is written and through which users extend it. The GNU Emacs Lisp Reference Manual describes it as a dialect of the Lisp programming language used as the extension language of Emacs, deeply integrated with the editing facilities so that programs can act directly on buffers, windows, and text. It is at once the implementation language of the editor and the scripting language exposed to its users, with no firm wall between the two.
This choice was deliberate and historically motivated. The original Emacs had been a set of TECO macros, and the experience of writing large extensions in TECO’s cryptic command language taught Richard Stallman that a serious editor needed a serious, readable language underneath it. Lisp, with its simple uniform syntax and its tradition of programs that manipulate programs, was the natural answer. When Stallman built GNU Emacs in the mid 1980s, Emacs Lisp became the foundation, replacing TECO entirely.
The payoff is the property Emacs is most famous for: it is self-documenting and infinitely extensible. Because each command is an Emacs Lisp function, the editor can show you the source and documentation of any command from inside itself, and you can redefine that command on the spot. The reference manual exists precisely so that users can read how the editor works and write new behavior. A keystroke, a mode, or an entire application can be added by evaluating Lisp, with no compile-and-restart cycle.
Two features of Lisp make this practical. First, code and data share the same list structure, so Emacs Lisp programs can build and transform other programs as easily as they handle text. Second, the language is interpreted and live: definitions take effect immediately in the running editor. Together these let a user treat the editor as a programmable environment, evolving it continuously rather than waiting for the next release.
The result is that Emacs is less a fixed program than a Lisp machine specialized for editing text. Decades of contributed Emacs Lisp have grown it into a platform for mail, version control, project management, and far more, all written in the same language as the editor itself. Emacs Lisp stands as the clearest demonstration of an idea central to free software: that giving users a real programming language inside their tools turns them from consumers into authors.