A Quick and Simple Dictionary

co-mother

1. Noun.  (obsolete) the relationship of a godmother to the other god-parents, and the legal parents, of a child. .
2. Noun.  in polygamy, a wife of one's father who is not one's mother; that is, co-wife of one's mother.
3. Noun.  in polygamy, the relationship between one co-wife and another, in respect to their children.
4. Noun.  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.