Eugene Volokh has suggested a new piece of socio-grammatical terminology ('Descriptivism, Prescriptivism and Assertionism", 10/4/2011):
Our readers likely know that I have many disagreements with prescriptivists when it comes to English usage. But while I have philosophical disagreements with prescriptivists in general, my main practical disagreements are with people who might best be labeled “assertionists” — people who don’t just say that prescriptions set forth by some supposed authorities define what is “right” in English, but who simply assert a prescription even in the face of what those supposed authorities say. Usage X is wrong, they say. Why? Because it violates this rule. What’s your authority for the proposition that this is a rule? Well, it violates the rule.
See also "If Only There Were No Assertionism", 10/22/2011:
Earlier this month, I blogged about assertionism — my label for usage claims that sound like prescriptivism, but are actually bare assertions: They don’t rely on any claims about what the (supposed) Linguistic Authorities say, on any detailed logical arguments, or on claims about allegedly superior clarity or precision; they just consist of a person’s bare assertions. And when one asks for evidence supporting the claim, all one gets is more bare assertions. Prescriptivists ought to dislike assertionism as much as descriptivists do, partly because assertionism often comes across as unintentional parody of prescriptivism.
Eugene supplies several fascinating examples of his own correspondence with assertionists about various topics.
My own experience, though, is that prescriptivists are assertionists more often than not. For every case of "this is so because the following style manuals and dictionaries say so", there are many cases of "this is so because this is so". Certainly, at least, that's what we see in our archives on Peeving and Prescriptivist Poppycock.
Update — in the comments below, UK Lawyer asks "Is an assertionist just a prescriptivist without footnotes?"
This is where it starts, in the examples that Eugene Volokh discussed, and also in many everyday interactions all around the English-speaking world. But there's an interesting difference that emerges when the assertion is challenged by information about facts of usage and opinions of reliable authorities. Is the response to withdraw the claim ("Oh, interesting, I guess I was wrong")? Or is it to double down with a series of other un-footnoted assertions ("But if you want to write really well, like Ronald Reagan or George Will, …")?
I've seen both, but the second type seems to be more common.