The Collaborative International Dictionary of English v.0.48:

Foster \Fos"ter\, a. [AS. f[=o]ster, f[=o]stor, nourishment. See
   Foster, v. t.]
   Relating to nourishment; affording, receiving, or sharing
   nourishment or nurture; -- applied to father, mother, child,
   brother, etc., to indicate that the person so called stands
   in the relation of parent, child, brother, etc., as regards
   sustenance and nurture, but not by tie of blood.
   [1913 Webster]

   Foster babe or Foster child, an infant or child nursed or
      raised by a woman not its mother, or bred by a man not its
      father.

   Foster brother, Foster sister, one who is, or has been,
      nursed at the same breast, or brought up by the same nurse
      as another, but is not of the same parentage.

   Foster dam, one who takes the place of a mother; a nurse.
      --Dryden.

   Foster earth, earth by which a plant is nourished, though
      not its native soil. --J. Philips.

   Foster father, a man who takes the place of a father in
      caring for a child. --Bacon.

   Foster land.
   (a) Land allotted for the maintenance of any one. [Obs.]
   (b) One's adopted country.

   Foster lean [foster + AS. l[ae]n a loan See Loan.],
      remuneration fixed for the rearing of a foster child;
      also, the jointure of a wife. [Obs.] --Wharton.

   Foster mother, a woman who takes a mother's place in the
      nurture and care of a child; a nurse.

   Foster nurse, a nurse; a nourisher. [R.] --Shak.

   Foster parent, a foster mother or foster father.

   Foster son, a male foster child.
      [1913 Webster]

WordNet (r) 3.0 (2006):

foster mother
    n 1: a woman who is a foster parent and raises another's child
         [syn: foster-mother, foster mother]