This is one of those funny nouns that, although singular, refers to a whole group of people. You know, like proletariat. The demimonde are a subsector of the proletariat. Working women. Prostitutes.
But you don't have to earn money to earn this label. The term can also be used to refer, collectively, to those women in society who have gotten a reputation for sleeping around.
It can also refer to any group who's activities are regarded with suspicion by society as a whole. Thus it can be seen that ROX is a sort of demimonde.
Any particular woman of the demimonde may be refered to as a demimondaine, which I'm sure you'll agree sounds classier than “whore” or “slut.” Nevertheless, it's not a very nice word to call somebody.
Demimonde can also be used in the singular to refer to a demimondaine, but I try not to think about that because I'm afraid my mind will be caught in some kind of infinite loop.
A demimondaine should not be confused with a demirep. A demirep is a woman suspected of sleeping around. Even though the two words are very similar and mean almost exactly the same thing, they are etymologically distinct!
Gosh, there sure are a lot of words in English for promiscuous women, aren't there. How come I can't seem to meet any?
That was a joke.
I don't think we have very many words for promiscuous men, however. If you haven't considered this before, think about it and draw your own conclusions. 'Nuff said.