Concubinage is the state of a woman in a relationship with a man of higher social status that gives her less rights than a marriage. In most of the cases the man has an official wife and one or more concubines. Concubines have only limited rights of support from the man, and their children have lower status than children born by the official wife or wives; these legal rights distinguish a concubine from a mistress.
Historically, the family of a girl were in favour of a concubinage with an mighty man, as it meant economic security for the woman. But concubinage could also mean sexual slavery of one member of the relationship, typically the woman, who was a pleasure slave to the man.
In the Western world there is no legal status of concubines, as only monogamous marriages are allowed. Any other relationship does not enjoy legal protection; the woman is necessarily a mistress.