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]

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "surrogate":
   advocate, agent, alter ego, alternate, alternative, amicus curiae,
   analogy, attorney, backup, backup man, champion, change,
   changeling, comparison, copy, counterfeit, deputy, double, dummy,
   equal, equivalent, ersatz, exchange, executive officer, exponent,
   fake, father figure, father image, figurehead, fill-in, ghost,
   ghostwriter, imitation, lieutenant, locum, locum tenens, makeshift,
   metaphor, metonymy, mother figure, mother surrogate,
   next best thing, paranymph, personnel, phony, pinch hitter,
   pleader, procurator, proxy, relief, replacement, representative,
   reserves, ringer, second in command, second string, secondary,
   sign, spares, stand-in, sub, substituent, substitute, substitution,
   succedaneum, superseder, supplanter, symbol, synecdoche,
   third string, token, understudy, utility man, utility player,
   vicar, vicar general, vice, vice-president, vice-regent,
   vicegerent



Bouvier's Law Dictionary, Revised 6th Ed (1856):

SURROGATE. In some of the states, as in New Jersey, this is the name of an
officer who has jurisdiction in granting letters testamentary and letters of
administration.
     2. In some states, as in Pennsylvania, this officer is called register
of wills and for granting letters, of administration in others, as in
Massachusetts, he is called judge of probates.