A concubine, in the past, was a woman who lived with a man and had a sexual relationship with him while that man already had a wife — or several wives. In some societies she had a recognised place in the household, but she stood lower than a wife: fewer rights, less honour, and her children often ranked below the wife’s children. You meet the word in history, old stories, and talk of royal courts — emperors, kings, and rich men in places where law or custom allowed this. It is not a modern word for a girlfriend, and it is not simply a secret affair. A mistress today is usually unofficial and hidden; a concubine, in the world this word describes, had a known rank, just not the wife’s rank. Though actually, English now uses it almost only about the past, or about historical drama. Calling a living person a concubine can sound cold or insulting, as if you are reducing her to a grade in a house. The word names the woman, not the man. Formal English keeps it; when students think of old Chinese courts, the identities they already know as 妾 or 嬪妃 are the ones this English word is reaching for.