GNU Emacs is an extensible, customizable, free/libre text editor.  At its
core is an interpreter for Emacs Lisp, a dialect of the Lisp programming
language with extensions to support text editing.

The features of GNU Emacs include:
  - content-aware editing modes, including syntax coloring, for many file
    types
  - complete built-in documentation, including a tutorial for new users
  - full Unicode support for nearly all human scripts
  - powerful customizability, using Emacs Lisp code or a graphical
    interface
  - a vast ecosystem of functionality beyond text editing, including a
    project planner, mail and news reader, debugger interface, calendar,
    even a psychotherapist, and more
  - a packaging system for downloading and installing extensions.

WWW: https://www.gnu.org/software/emacs/