concubine
dictionary.cambridge.org
concubine
noun [ C ]
UK /ˈkɒŋ.kjə.baɪn/ US /ˈkɑːn.kjə.baɪn/
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a woman who, in some societies, lives and has sex with a man she is not married to, and has a lower social rank than his wife or wives
妾,姨太太,小老婆