Multiple realities
(covers information from several alternate timelines)
You may be looking for the title, mistress.

A mistress was a man's long-term female sexual partner and companion who was not married to him, especially used when the man was married to another woman.

In Sherlock Holmes Program 3A, the King of Bohemia had a mistress. A photograph of both parties together was intended to be used as blackmail. (TNG: "Elementary, Dear Data")

In the mirror universe, Jadzia Dax was a Trill soldier of fortune who worked for the Terran Rebellion. She was the lover/mistress of Benjamin Sisko. (DS9: "Through the Looking Glass")

Gul Dukat, like many Cardassian officers during the Occupation of Bajor, conducted numerous affairs with Bajoran women, and had at least two mistresses, including Kira Meru, mother of Kira Nerys, and Tora Naprem, whom he claimed to love, and with whom he fathered a daughter, Tora Ziyal. (DS9: "Indiscretion", "Wrongs Darker Than Death or Night") Damar also had a mistress. (DS9: "Penumbra")

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