Concubinage is either the state of a couple living together as lovers with no obligation created by vows, legal marriage, or religious ceremony, or the state of a woman supported by a male lover who is married to, and usually living with, someone else.
In the Western world, this term has become antiquated and has been replaced by the terms "living together", "shacking up", or, more formally, cohabitation if both members of the couple are otherwise unattached. The term mistress is used for a woman who has an ongoing sexual relationship with a married man. Traditionally, in the Western world, a man and a woman who lived together without being married were socially shunned; in some jurisdictions, this was even illegal.
The term concubine is generally used to signify an outdated relationship model where the male is the dominant partner, socially and economically. In this model the male may also have one or more wives in addition to the concubine.
Another definition is a woman who is attached to a man solely for reproduction, and caring for the resulting children without any romantic relationship. The father in this case may have a legal wife or other lovers.
Concubinage was frequently voluntary as it provided a measure of economic security for the woman involved in the relationship. When involuntary, "servile concubinage" involves the chattel slavery of one member of the relationship, most typically the woman.