The Collaborative International Dictionary of English v.0.48:

Surrogate mother \Sur"ro*gate mother\, n.
   1. a person or who serves in place of the biological mother
      for child, as for caring or nurturing.
      [PJC]

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

   3. 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.

   Note: The procedure is common but controversial, and in some
         states of the United States, contracts for surrogacy
         are not considered enforceable.
         [PJC]

WordNet (r) 3.0 (2006):

surrogate mother
    n 1: a woman who bears a child for a couple where the wife is
         unable to do so; "a surrogate mother is artificially
         inseminated with the father's semen and carries the fetus
         to term"