[tadfisher]

Emacs is a shell, not an OS. It's an extremely customizable and programmable shell that works with many third-party utilities, but it's still a shell.

[rbc]

I'd call it more than a shell. It's more like a text oriented Lisp (Emacs Lisp to be specific) runtime environment with built in editing, interpretation, compilation, and debugging tools. It's also been a popular home for numerous applications that have found lives of their own, far beyond editing text.

[hollerith]

I like to call it a generalized interface to information.

The web of the early 1990s could be called the same thing, but not the modern web, which is more like a "generalized experience-delivery platform with an old much-simpler generalized interface to information at its core".