Emacs is a text editor whose defining feature is that it can be reprogrammed by its users. GNU Emacs, the canonical free version written by Richard Stallman, is described on its project page as “an extensible, customizable, free/libre text editor, and more.” It became one of the first major pieces of software released by the GNU Project.
What makes Emacs extensible is that at its heart lies “an interpreter for Emacs Lisp, a dialect of the Lisp programming language with extensions to support text editing.” Users can change how the editor behaves by writing Emacs Lisp code, and a packaging system lets them download and install extensions. This design lets Emacs grow well beyond editing into tasks like reading email, debugging, and project planning.
Emacs also emphasizes self-documentation. The project page notes it ships with “complete built-in documentation, including a tutorial for new users,” so the editor can explain its own commands and features from within.
The tool sat at the center of the early GNU effort. Stallman’s 1983 GNU announcement listed an editor among the first utilities the free system would need, and GNU Emacs, which he made usable in early 1985, helped fund and define the Free Software Foundation’s work.