surrogate mother

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A woman who agrees, often for pay, to give birth to a child resulting from artificial insemination or the implantation of an already fertilized egg and who surrenders any parental rights to a third party.
• noun One that acts as, serves as, or is a mother substitute.

from the GNU version of the Collaborative International Dictionary of English.

• noun a person or who serves in place of the biological mother for child, as for caring or nurturing.
• noun a female animal that raises and nurses the offspring of another animal.
• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A woman who gives birth to a child not her own, with the intent to give that child up to at least one of the biological parents once it is born.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a woman who bears a child for a couple where the wife is unable to do so