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 societies which permit slavery, it often refers to sexual relationships between slave-owners and their slaves; a variety of sexual slavery.
In the Western world, this term has become antiquated and has been replaced by the terms "living together", "shacking up", or, more formally, "cohabitation," if both members of the couple are otherwise unattached.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. Today, cohabitation is a common (if not a majority) pattern among younger people, especially those who desire marriage but whose financial situation temporarily precludes it, and perhaps most couples who marry have lived together beforehand, possibly before the engagement, to the point that cohabitation has become known in some circles as "college marriage."
Concubinage is however used as an official term in France for describing the situation of a couple (heterosexual and, since 1998, homosexual) that lives together. Some benefits of couples bound by marriage or PACS (civil union) may then apply. In jurisdictions with common-law marriage, romantic partners may be considered common-law spouses after cohabiting for a certain length of time.
The term concubine is generally used to signify a 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. Some historical Middle Eastern rulers are examples of this pattern.
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 or sexual slavery of one member of the relationship; typically the woman.