In a lesbian couple, the nonbirth mother (partner of the birth mother) of a child, especially one who takes an equal role in mothering the child.
In polygamy, the relationship between one co-wife and another, in respect to their children.
In polygamy, a wife of one's father who is not one's mother; that is, co-wife of one's mother.
A woman who gives birth to a child not her own, with the intent to give that child up to at least one of the biological parents once it is born.