妾，又称姨太、陪房、偏房、侧室、副室、妾侍、小妻、旁妻、下妻、庶妻等，亦有小老婆的俗称，以及如夫人的雅称，主要指一夫一妻多妾制结构中，地位低于正妻的女性配偶。

周代贵族女子出嫁，需要同族姐妹或姑侄陪嫁，称为媵，媵会成为侧室，地位比妾高。后世媵和妾渐渐不分。

《礼记》：

天子之妃曰“后”，诸侯曰“夫人”、大夫曰“孺人”，士曰“妇人”，庶人曰“妻”。

公侯有夫人，有世妇，有妻，有妾。"

天子有一后、三夫人、九嫔、二十七世妇、八十一御妻

汉代以后一般的士大夫、平民都只有一个正妻（正室），同时可拥有多个妾。多数正妻是丈夫的第一位配偶。但也有“先纳妾后娶妻”的例子，如章太炎于1892年纳妾王氏，王氏去世后与汤国黎结婚，但只称王氏为妾。

Concubinage is an interpersonal relationship in which a person engages in an ongoing sexual relationship with another person to whom they are not or cannot be married to the full extent of the local meaning of marriage. The inability to marry may be due to multiple factors such as differences in social rank status, an existent marriage, religious prohibitions, professional ones (for example Roman soldiers) or a lack of recognition by appropriate authorities. The woman in such a relationship is referred to as a concubine.