co-mother

noun4

1. •
   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.

   • The dilemmas engendered by the absence of a biological tie between a child and co-mother illuminate the centrality of familial rights and obligations in American kinship. — 2004
2. •
   In polygamy, a wife of one's father who is not one's mother; that is, co-wife of one's mother.

   • the two of them went to their mother's forest hut and tried to find a way to avenge their mother's death and kill their co-mother. — 2001
3. •
   In polygamy, the relationship between one co-wife and another, in respect to their children.
4. •
   (no longer used) The relationship of a godmother to the other god-parents, and the legal parents, of a child.

   • It is considered incestuous for a co-mother and co-father to have sex relations — 1967