The Collaborative International Dictionary of English v.0.48:

Surrogate \Sur"ro*gate\, v. t.
   To put in the place of another; to substitute. [R.] --Dr. H.
   More.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Surrogate \Sur"ro*gate\, n. [L. surrogatus, p. p. of surrogare,
   subrogare, to put in another's place, to substitute; sub
   under + rogare to ask, ask for a vote, propose a law. See
   Rogation, and cf. Subrogate.]
   1. A deputy; a delegate; a substitute.
      [1913 Webster]

   2. The deputy of an ecclesiastical judge, most commonly of a
      bishop or his chancellor, especially a deputy who grants
      marriage licenses. [Eng.]
      [1913 Webster]

   3. In some States of the United States, an officer who
      presides over the probate of wills and testaments and
      yield the settlement of estates.
      [1913 Webster]

   4. a surrogate mother.
      [PJC]

WordNet (r) 3.0 (2006):

surrogate
    adj 1: providing or receiving nurture or parental care though
           not related by blood or legal ties; "foster parent";
           "foster child"; "foster home"; "surrogate father" [syn:
           foster, surrogate]
    n 1: someone who takes the place of another person [syn:
         surrogate, alternate, replacement]
    2: a person appointed to represent or act on behalf of others
       [syn: deputy, surrogate]