Emacs is a programmer’s text editor that can be extended and configured using it’s embedded scripting language ELisp (Emacs Lisp).
There are multiple members of the Emacs editor family, the most popular one being GNU Emacs. These have varying levels of compatibility with each other, ranging from “mostly compatible” to “some key bindings are the same”.
This is the most common Emacs “distro”.
This is an Emacs clone that is included with OpenBSD. The source code for the mg editor can be found at ibara/mg.