Concubinage, term, no longer used in the Western world, that refers to the cohabitation of a man and a woman without sanction of legal marriage. Specifically, concubinage is a form of polygamy in which the primary matrimonial relation is supplemented by one or more secondary sexual relationships.

Concubinage was a legally sanctioned and socially acceptable practice in ancient cultures, including that of the Hebrews; concubines, however, were denied the protection to which a legal wife was entitled. Ancient Germans also accepted the practice as an inferior form of marriage. In Roman law, marriage was precisely defined as monogamous; concubinage was tolerated, but the concubine's status was inferior to that of a legal wife. Her children had certain rights, including support by the father and legitimacy in the event of the marriage of the parents. The early Christian church upheld the sanctity of monogamous marriage, but the ecclesiastical leaders found it necessary to deal with the problem of concubinage on several occasions, particularly in connection with the enforcement of clerical celibacy. Concubinage was an accepted practice in Muslim cultures. In Muslim harems concubines had no legal status, but their children had some rights of inheritance.