n. A man who has an extramarital affair with a woman.
Not every woman is currently in pursuit of a white-picket fence lifestyle, relationship of permanence, children, mortgage and scheduled date nights," says Noel Biderman, founder and CEO of AshleyMadison.com. "Many women are seeking adventure, and on a service like ours encountering older sophisticated men with the same 'no strings attached' mentality that creates the perfect match. Also it is very evident that this is not just a female thing, there are an increasing number of single men on AshleyMadison.com connecting with married women…we refer to them as 'MISTEResses'.
—“Dubious website names Los Angeles top most mistress city in Nation,” Guardian Express, November 03, 2012
Mr. NEUMAN: I even say, you know, we call her a mistress, what do you call the man that she cheats with?
KOTB: Why are…
Mr. NEUMAN: There's not even a word in our vocabulary.
GIFFORD: Oh, we need a word.
KOTB: We know—we understand why some men cheat, what are you hearing about why…
GIFFORD: Misteress. Misteress.
KOTB: Misteress.
Mr. NEUMAN: Misteress. Thank you.
—“'Connect to Love' author M. Gary Neuman discusses why some women cheat on their husbands,” Today, January 19, 2011
2004 (earliest)
DEAR ANNIE You recently printed a letter from "Equal Rights in Michigan." She said a woman who has an affair with a married man is called a "mistress," but what do you call a man who has an affair with a married woman?…

Houston: How about "misteress"?
—Kathy Mitchell & Marcy Sugar, “Annie's Mailbox,” South Coast Today, March 30, 2004
Thanks to reader Glenn Bourdot for spying this term.