Except the OP said he didn’t want to use that term.
I had a similar situation. “My dad’s wife” always worked just nicely. Nothing against the lady, but she was not my stepmother, as I was about 50 when he remarried. I ain’t got no stepmother. I don’t need no stepmother. I don’t need no stinkin’ stepmother!
You could make up a silly name. I call my FIL’s wife ‘‘Squashmother.’’
If that’s too weird for you, you could always come up with some more dignified unique signifier.
“Carol”.
Well, assuming her name is Carol…
My wife’s parents divorced when my wife was a teenager. When I started dating her, both her father and her mother were on spouse #3, and both marriages had happened when my future wife was an adult. She referred to her mom’s husband as her stepfather, and her dad’s wife as her stepmother, despite the fact that neither of them had any role in raising her.
Her stepfather’s mother was still alive at that time (and, in fact, she lived to nearly 100, so I knew her for many years). She was a lovely, funny woman, and I remember we had her over for her birthday one year. I joked with her that I had a difficult time finding a birthday card for her, as the “Step-grandmother-in-law” section at Hallmark was very limited. 
Since the OP is seeking opinions, let’s move this to IMHO.
Colibri
General Questions Moderator
Yo call her “my father’s wife.” She is not your stepmother.
That has been my situation for the past 40+ years, and this is exactly how I spoke of her.
She was always [her first name] or [Mrs. last-name] as appropriate, and whenever it was relevant to the conversation that she was my father’s wife, she was “my father’s wife”.
Seems like calling her “my father’s wife” includes a judgement that you don’t like her and want to make clear that you aren’t interested in any sort of relationship with her.
My FIL has made marriage and divorce an Olympic event. I call his current wife (with her full knowledge and blessing) “MIL #5”. We joke that numbering is easier than learning a new name every few years.
I read it now as, and for years used it as, completely value neutral. The familial wiring diagram between any two people says exactly zero about what you think of them or they think of you.
To me your POV sounds as sensible as “You call her ‘Carol’. I don’t like the name ‘Carol’. Therefore you must not like the woman.” IOW, makes no sense.
Mothering and fathering is about bearing, nurturing and raising children. Husbanding and wifing is about adults in committed cohabitation. IMO they’re orthogonal concepts although admittedly the traditional standard was to treat one as a prerequisite for the other.
*If *everybody always did the husband / wife thing with exactly one other person per lifetime *and *then the happy couple did the mother / father thing, *then *this would all be easy and “father’s wife” and “mother” would be perfectly synonymous spots in the wiring diagram and we could use the terms interchangeably or use them for different tints of approval or affection or whatever.
But that’s emphatically not the world we live in.
I like, “that woman.”
Well, a friend of mine uses “stepmother” even though the woman in question is a year younger than she is. And she uses “ex-stepmother” for the one before that. I don’t know how old that one is, but they still have a relationship even though she’s not married to my friend’s father any more.
That’s how my former MIL referred to the ex-wife of the son who was not my husband.*
It’s hard to get the sneer in there, though.
Correct usage does not depend on feelings.
And for the record, I have never one regarded stepmother as including maternal components.
Although most are fine there are a few dodgy stepmothers, along with wicked uncles, in adequate fairy-tales and Märchen.
For that matter, considering the variety, I should not automatically assume mothers to every one be maternal in instinct.
Well, when calling her by name simply won’t do…
I’d call her stepmother.
Those close to me know what the score is. And if those not so close to me get the wrong impression; I don’t think I’d really care.
ETA: Actually, it really depends on the conversation.
Sometimes she’s dad’s wife, sometimes she’s Carol and sometimes she’s stepmother.
There really is no blanket answer.
I would say “my father’s wife” but that is more from a personal comfort level. Use whatever means/words feel right to you; especially if we’re talking out of her presence in conversations with other. When around her, I would talk it over and see if some mutual understanding could be reached.
Well, the denotation is that she is indeed his stepmother. You can argue that the term might imply certain connotations, but you can’t dispute the fact.
My aunt has been married somewhere between seven and nine times. The other sister was married four or five times. I hope people won’t fault me for forgetting exact numbers. My father wasn’t close to either of them, they lived in California and we grew up in Utah so we would get updates in the Christmas newsletters.
They were simply called “uncle” without a name attached, lest we make mistakes.
I would say that is “technically correct usage” as many people in this thread have indicated that “stepmother” implies that the husband’s wife was around when the person was growing up. It has less to do with feelings than implied meanings.
You didn’t understand the problem. Within the family, whether she is there or not, she is called and referred to as Mary* - we all know who and what ‘Mary’ is. My problem was that if I wanted to relate some anecdote about her to a stranger, I felt that calling her “stepmother” implied a relationship that was not accurate.
I am happy with “father’s wife” I think.
I’m 59. My mom died 5 years ago, my dad a few years prior. While I’m technically now an orphan, I think people would object to my showing up at the orphanage looking for assistance or housing.