Wiktionary
other woman
n. (context informal English) A woman who is romantically involved with someone (usually a man) already in a committed relationship with another woman, especially a woman having an affair with a married man.
Usage examples of "other woman".
She squatted beside the other woman, poking through the little pile of artifacts.
Miss Robbins couldnt help but feel a little glad at the discomfiture of the other woman.