1. a person who acts in the place of another person's biological mother.
2. an animal that is given another's offspring to raise.
3. Med.a woman who helps a couple to have a child by carrying to term an embryo conceived by the couple and transferred to her uterus, or by being inseminated with the man's sperm and either donating the embryo for transfer to the woman's uterus or carrying it to term. Cf. embryo transfer.