PhD in comparative literature
I first read this as 'competitive literature', which is probably apt for this thread. Only on Mumsnet.
I agree with the poster who said 'I still maintain that it is not a common word at all and that the vast majority of people would not be familiar with it even many people who are well read with a solid vocabulary'.
At the start of the thread I had no idea what the word meant and was fairly certain I'd never heard or seen it used, despite reading a proper newspaper, reading for pleasure and watching the news regularly and being a professional who works with many other professionals who are worldwide experts in their technical/scientfic field and meeting lots of engineering and scientific types professionally.
Having read the whole thread and gone through thinking it means 'something like emollient, is a technical term used in geography and a few other things, I've now learned that it means 'mitigate' (I think) and certainly when writing in a professional capacity, whether for scientific colleagues, clients who are generally engineers or scientists or members of the public, I wouldn't use that word, because it certainly wouldn't pass any kind of 'plain English' test and would be struck out by those approving my work.
I've learned a new word this morning, but I don't think it's one that I'll be using any time soon.