foster mother

a woman who takes the place of a mother in raising a child.

a nurse.

Origin of foster mother

before 1000; Middle English; Old English

Garth cannot make scenes before his foster-mother, for the very shame of it!

Vision House |C. N. Williamson
There are three common methods of rearing calves, The calf sucks its mother or foster-mother.

Encyclopaedia Britannica, 11th Edition, Volume 5, Slice 5 |Various
This was my brother William, then nine months old, and Margaret Spence was his foster-mother.

Spare Hours |John Brown
Twice a year his foster-mother's brother, the eel-man from Fjaltring, up near Rovbierg, paid them a visit.

The Sand-Hills of Jutland |Hans Christian Andersen

And he came through the wood and knocked at his foster-mother's door.

Moonshine & Clover |Laurence Housman

foster mother

a woman who looks after or brings up a child or children as a mother, in place of the natural or adoptive mother