It's tempting to accept the view of Tom and others that the common/rare distinction is totally subjective, and state that
diorama is quite well known to me, so it's common - end of story.
Since I'm still undecided about how to respond to this request, I'll digress for the moment, and consider this notion of subjectivity. The question of whether I know the word
diorama is not what I would call subjective. It's a factual issue. I have clear memories of seeing many dioramas as a youngster in the Melbourne Museum, when it was in the same building as the State Library. I now see that the word has been used for a few different ways of representing scenes, but this is the sense of the word that I knew:
Museum dioramas are three-dimensional life sized or scaled down models usually depicting a natural scene or historical event for the purpose of education and entertainment. In most cases they employ a painted backdrop combined with realistic foreground to create a trompe l'oeil effect, evoking the illusion of a real scene.
This is from a page on the Museums Victoria website,
The art of the diorama.
I knew as a child that they were called
dioramas, and I never forgot it. If someone had shown me one and asked what it was called, I would have said it was a diorama.
But is my knowledge of the word likely to be shared by other Chihuahua players, current and potential? This is a difficult question to answer, but I think it is also a question of fact. It would be pointless for me or anyone else to answer it subjectively, by saying, "It seems to me like the sort of word that almost everyone would know," or "It strikes me as the sort of word a lot of people wouldn't know." We need to look for evidence of people's knowledge or ignorance of the word. Or where that is unavailable, we can at least try to infer how well the word is known from evidence of its usage.
The same Museums Victoria page says, "At the height of their popularity in the 20th century, museum dioramas could be found in almost every natural history museum, both locally and internationally, and in a variety of shapes, forms and genres." This might imply that the word, at least in this sense, is dropping out of use.
But it appears by many yardsticks to be in quite wide use still. It seems to be in every dictionary, however small. It isn't queried by spell-checkers as far as I can see. It's in both the "2of12inf" and "2of4brif" word lists that I made use of in defining the common word list in the first place - lists derived from English learners' dictionaries.
As far as usage is concerned, the corpus of Global Web-Based English shows Australia as the country where diorama is most often used, while the News on the Web (NOW) corpus shows the United States as the most frequent user. Nevertheless, both corpora have plenty of British examples too. NOW has 295 citations, from the
Guardian,
Telegraph,
Daily Mail,
Evening Standard,
Financial Times, etc, etc. As well as references to physical models, the word is sometimes used in descriptions of video games: "Tokyo 42 is a little marvel, a challenging adventure that mixes slick hyper-violence, stealth and exploration into its gorgeous diorama." ("The best video games of 2017", Telegraph, 4 December 2017)
But at the same time, anona tells us that she didn't know of the word, and MK says he barely knew it. It was played by fewer people than any other common word in that puzzle. But of course there always has to be some word that has the fewest hits.
On balance I'm going to leave it as a common word for now.