“Person in a relationship I disapprove of” is, I’m starting to sense, the answer you’re looking for here.
Girlfriend, ladyfriend or SO.
What would you call the woman in this relationship.
I’d call her not too bright or very lucky depending on the circumstances.
How do they refer to her in polite company? Use that.
Paramour???
Mrs Q.
[QUOTE=Hershele Ostropoler]
“Person in a relationship I disapprove of” is, I’m starting to sense, the answer you’re looking for her
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:rolleyes: My approval or disapproval is undoubtedly irrelevant to them and certainly is to this thread. You “sense” seems to be misfiring.
How about “companion”?
[QUOTE=even sven]
Concubine" implies an arranged sexual relationship where the woman plays a very passive role. Historically, concubines have basically been property, without choice in the matter and without other prospects in life. Nobody wants to be a “concubine”, and nobody wants to think of their partner as that.
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Concubine strictly meant a woman in a relationship with a man which was permanent, long term, similar to marriage but without the formal recognition of that state. I agree its a loaded term, but it does describe what this is.
I don’t really talk about people love life at their or anyone elses dinner party or in most situations really. 
Why are you so invested in this? Why are you even thinking about it?
They stare at you because they wonder if you just fell down a rabbit hole from the 1880s.
No, it doesn’t. Context is a thing. History exists. Words have connotations other than their dictionary definitions. Please keep your weirdo brain thoughts out of this woman’s business, and find something else to focus on.
A “concubine” is practically a slave. It hardly describes the relationship you find so odd. There’s no reason for you to be privy to the details of that relationship. In fact, your disapproval makes me wonder why they bother with you at all. Who do *you *bring to those social events?
How about “girlfriend”? You rejected a perfectly respectable term for an arbitrary reason, unless the real reason is that you don’t want a respectable term.
I never use the term “girlfriend” for people older than their early twenties. It doesn’t sound right.
Really SO - significant other is the best term.
Lady Friend works for me.
He calls her Honeybunch, and she call him Snookums.
Looking at the Wiki article that doesn’t appear to have been so, although some were. It looks more like something that doesn’t exist anymore because we don’t practice polygamy and no longer have such rigid class barriers; if some modern rich guy falls in love with a poor woman he has no legal barrier to just marrying her.
How about “old lady”?
What a useful term.
At a place I used to work at, one of my co-workers used to called the people in those circumstances “common-law bed-mates”. I thought the term was insulting, but in some circumstances, somewhat accurate.
If you want to go for strict definitions, both the OED and Merriam-Webster define a concubine as a woman who cohabits with a man but isn’t married to him. So if Mr. X and Mrs. Q don’t live together, she’s not his concubine. Even if they did live together, calling Mrs. Q a concubine sounds like you’re grasping for either a more exotic way of saying she’s Mr. X’s mistress or a more polite way of saying she’s his whore.
I referred to an ex-girlfriend as my concubine once, semi-facetiously but not inaccurately. She was furious after she looked it up. It was more loaded than I thought.
**AK84 **are they exclusive? I can see your dilemma because none of the words you suggest are really apt, and I’d hesitate to say that “SO” or “girlfriend” are apt if their relationship isn’t exclusive. Although I suppose it may depend on what you put before those words ie if someone asked you “what relationship does this guy have with Mrs Q?”, much would depend on whether you said:
"She is a girlfriend [or “an SO]” or
“She is his girlfriend [or “an SO”]”
The former is very pointed and the latter, to me, strongly implies exclusivity. You haven’t said whether they are exclusive.
I must say I also don’t get from “girlfriend” the right mental picture because I don’t see the word as quite apt for someone you have two children with.