I find this to be unnecessarily prescriptive, as are almost all purely semantic assertions about what should be (semantic used in its weak, normal language sense here). It is certainly true that Schemes and Clojures and Adders and Emacs Lisps and so on differ in many respects from "Common Lisp" but that hardly justifies fixing the label "Lisp" to refer only to Common Lisp. I mean _Emacs Lisp_ contains Lisp in its name! Would you claim Emacs Lisp is not a lisp?

It remains that these languages have more in common with one another they they have in common with, say, Ada or Standard ML, but even were it not the case, so long as a listener understands what a speaker means when they refer to a language as being "a lisp," I see no problem with the continued usage. Language works when it allows communication, and as long as a word clearly denotes a concept in common parlance, it hardly needs to be dictatorially controlled. If people were referring to Clojure as a "Common Lisp," it would be useful to correct them, but this is not the case.

Vague language can even be enlightening, as it forms the nucleating body for useful discussions. For instance, one might pose the question "Is Ruby an Acceptable Lisp?", and while the answer is obviously, stridently, hilariously, no, the discussion will help illuminate what both terms mean.

So I favor letting language do its job, and letting lisp continue to refer, as an improper noun, to a cloud of languages sharing a family resemblance.