Concubinage is the state of a woman or man in an ongoing, usually matrimonially oriented, relationship with somebody to whom they cannot be married, often because of a difference in social status or economic condition.

A concubine is generally a woman in an ongoing, marriage-like relationship with a man with whom she cannot be married for a specific reason. It may be because she is of lower social rank than the man (including slave status) or because the man is already married. Generally, only men of high economic and social status have concubines. Many historical rulers maintained concubines as well as wives.

Historically, concubinage was frequently voluntary (by the woman and/or her family's arrangement), as it provided a measure of economic security for the woman involved.

Under Roman law, Roman culture under the Empire came to tolerate concubinage so long as the relation was durable and exclusive; for the jurists, concubinage was an honourable de facto situation.

When having no legal status but being recognized, or defined in law, as in ancient China, concubinage was akin, although inferior, to marriage. The children of a concubine were recognized as legal offspring of the father; their inheritance rights may have been inferior to younger children of a marriage, or they may have received a smaller inheritance. Men frequently used concubines to bear heirs when he and his wife were unable to produce sons.