Wet-Nurse
A wet-nurse is a woman who breast-feeds somebody else's baby, either for money or because she is a slave.
In those days they did not have baby formula to put in bottles, so it was much healthier for babies to be breast-fed, even if it was not their own mother's milk, than it was for them to have plain cow's milk (which wasn't available all year round anyway). Besides, without refrigeration or pasturization cow's milk often had germs in it which could kill babies or make them sick.
It was very common throughout Europe and Asia for rich women to have wet-nurses for their babies, so they would be free to run businesses or to rule countries.
Even poor women often had wet-nurses so that they could go back to work in the fields or as house servants.
To find out more about wet-nursing, check out these books from Amazon.com or from your library:
Wet Nursing: A History from Antiquity to the Present, by Valerie Fildes (1988, currently out of print).
Infants, Parents and Wet Nurses: Medieval Islamic Views on Breastfeeding and Their Social Implications, by Avner Giladi (1999).

