A computer built totally on Lisp. Even its processor ran a form of Lisp.
This concept is also known as Lisp all the way down meaning there is Lisp at every level of the system.
Some people have started to refer to Emacs as a LispMachine. It is not strictly a LispMachine because, clearly, it is a software program and not a physical computer, but as that line starts to blur it seems like a useful epithet for Emacs because these days Emacs look more and more like an operating system.
Emacs as an OperatingSystem? is, in fact, a common joke Emacs is a great operating system, it just doesn't have a good editor.