n. The current wife of a woman's ex-husband; the ex-wife of a woman's current husband.
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We never really knew what to call each other," Ms. Oxhorn-Ringwood said. "When Evan was a kid, I would call Louise my ex-husband's new wife, but after 10 years I couldn't do that anymore. We came up with stepwives to describe the relationship between ex-wives and current wives.
—Alex Witchel, “Wives No. 1 and No. 2 Bury the Sandals,” The New York Times, May 12, 2002
The premise of Phillis Stevens' first novel, STEPWIVES (Crown, $18.95) — it's O.K. to murder your husband if the guy's a slob and you need the money — is enough to thin your blood. The comic scheme to assist the Santa Fe millionaire Mac Carpenter into an early grave is hatched when the first Mrs. Carpenter (who has become her former husband's housekeeper) persuades another penurious divorcee to become the second Mrs. Carpenter. Between her own high-cholesterol cooking, reasons Marilee, and Karen's bedroom duties, fat and greedy Mac will keel over with the heart attack he deserves.
—Marilyn Stasio, “Crime,” The New York Times, August 26, 1990
1989 (earliest)
We asked for specific phrases for adults to use instead of "boyfriend" and "girlfriend." Here are some of your suggestions: … Nikki Klinsky — "Stephusband" or "Stepwife."
—Bob Greene, “It's a close call among old friends,” Chicago Tribune, January 24, 1989
This word is in the news because of a book called Stepwives: 10 Steps to Help Ex-Wives and Stepmothers End the Struggle and Put the Kids First, by Louise Oxhorn and Lynne Oxhorn-Ringwood. The first citation hints that they coined the term, but that's not true. However, the earliest citation I could uncover for this word isn't quite right, as you can see below.

In case you're wondering, the word stephusband — the current husband of a man's ex-wife or the ex-husband of a man's current wife — has made the occasional appearance in the media over the years. The earliest citation for this sense is the same as the one for stepwife.

In our context, the woman must be a man's wife or ex-wife and not merely a girlfriend. So the honor of coining the term in its "correct" sense goes to writer Phyllis Stevens who published a novel called Stepwives in 1990.