I’m one of those who like to say: “Emacs is not a text editor!” And when someone asks me what it is, I usually reply: “Emacs is a poor man’s Lisp Machine.”

The old idea of an Integrated Development Environment (IDE) was quite different from modern IDEs. It used to mean a runtime environment (almost an operating system) where everything was written in the same language, where you could tweak anything and hot-reload code at will. In such an environment, every program you wrote was “integrated” with the others.

In that sense, GNU Emacs is an extensible Lisp environment, whose philosophy is inspired by old Lisp machines and systems like Symbolics Genera.