surrogate mother

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A woman who bears a child for another person, often for pay, either through artificial insemination or by carrying until birth another woman's surgically implanted fertilized egg.
2. n. One that acts as, serves as, or is a mother substitute.

Wiktionary

1. n. 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.

GNU Webster's 1913

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

WordNet 3.0

1. n. a woman who bears a child for a couple where the wife is unable to do so