a person or who serves in place of the biological mother for child, as for caring or nurturing.  [PJC]

 a female animal that raises and nurses the offspring of another animal.  [PJC]

 a woman who becomes pregnant by an artificial procedure (one other than copulation), and bears a child to be raised by another person. The person on whose behalf the child is gestated and born is usually one of the biological parents. The embryo carried by the surrogate mother may be created by in vitro fertilization, and may have no genetic material from the surrogate mother, or the surrogate mother may be impregnated by artificial insemination, usually by sperm donated by the father who is to raise the child.