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

Social status

In sociology or anthropology, social status is the honor or prestige attached to one's position in society . It may also refer to a rank or position that one holds in a group, for example son or daughter, playmate, pupil, etc....

 or economic condition.
A concubine is generally a woman in an ongoing, marriage-like relationship with a man whom she cannot marry 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.